Abstract
This work presents a review of the ideas that are currently in use on the ontology-based conceptual modeling of occurrents (sometimes referred to as “events”, “perdurants”, or “processes”). It collects such ideas from a set of 11 ontologies, which includes some of the most important and widely used upper ontologies (i.e., BFO, UFO, DOLCE, YAMATO, SUMO, GFO). We analyze the ontologies with respect to the definition of occurrent they present and their understanding about participation, mereology, and causation. The commitments regarding these four facets of occurrents are gathered in three categories (pervasive aspects, complementary aspects, and conflicting aspects). Additionally, we identify the main occurrent classification criteria used to branch the taxonomy of the ontologies. These findings are summarized in two tables at the end of the paper, which may be used by modeling practitioners as reference. The review shows that the considered ontologies agree in a significant set of common aspects as well as present some relevant divergences. However, there is a considerable set of non-conflicting, complementary aspects scattered among the diverse ontologies. It suggests an opportunity for efforts aiming to harmonize those views in a single approach that may enrich the analysis and representation of occurrents.
Introduction
If we take a minute to picture in our minds the world that surrounds us, the resulting image will certainly be composed by a variety of objects (e.g., the Sun, clouds, people, buildings). In summary, this picture will be crowded by continuants or endurants – i.e., things that exist in time, being wholly present at any time they are present, keeping their identity while enduring change over time. For a moment, one may think that the whole world could be described solely by things of this kind. Nevertheless, thinking a little further, we promptly realize that, along with the things that exist in this picture, there is a lot of things happening on it – the numerous events the objects suffer, perform or take part in (e.g., a sunrise, clouds passing through the sky, people attending to meetings, buildings being repaired). In other words, it seems not possible to have a complete account of the world without considering its occurrents or perdurants – i.e., things that happen in time, being only partially present whenever present (which sometimes are called “events” or “processes”).
Besides this initial appeal to common sense towards the existence of events, there is evidence that humans present a commitment to the existence of such things as occurrents. We seem to be able discriminate events even since pre-linguistic childhood, we plan and carry out actions in order to bring about changes in the world, and we have certain linguistic apparatus (e.g., verbs, nominalization of verbs) carefully adapted to refer to events (Casati & Varzi, 2015). Moreover, without assuming the existence of events, it would be much harder to make sense of our talk and reasoning about actions (Davidson, 1967a, 1969) and even of much of our most ordinary talk (Davidson, 1967b). Along with that, events are also regarded as targets of explanation and relata of causal relations, being a key part of such theories (Kim, 1969). Having that in mind, any model that intends to depict the full complexity of the world must take into account both continuants and occurrents.
This is particularly important in informatics. The relevance of modeling occurrents in computer systems is growing with the amount of information that is shared in open systems. For example, the internet has become a large repository of information describing states of a same entity in distinct moments. With that, recovering reliable information became a challenging task, that may take advantage of the capability of processing occurrents and the modifications they cause over continuants. For example, if we search for Arnold Schwarzenegger, we will find him as an actor, a professional bodybuilder and the Governor of California. Knowing which of these facts are true about him now requires understanding the events that led through such states (e.g., retiring sports activities ended up his bodybuilder career; leaving Governor’s Office he is no longer a governor).
Thus, this work presents a review of ideas that are currently in use to represent occurrents, collected from a set of ontologies found on literature. A great deal of work has been carried out on the Ontology of Occurrents within the realm of Philosophy, that may undoubtedly provide invaluable insights on the development of ontologies for the representation of occurrents on informatics related fields. Despite that, this survey focuses on works that are explicitly conceived to deal with issues related to Conceptual Modeling, Knowledge Representation, and related areas – in other words, to meet the needs of applications in informatics. The underlying goal is to present a review of the ideas grounded on a somewhat reasoned basis, that have possibly originated on philosophical works, but that are put to the test of facing the challenges of the practice of ontology development/analysis (i.e., that were already proposed by some ontology). Moreover, we will consider the notion of occurrent as that kind of thing that happens in time, including either those referring to some kind of change in the world (e.g., a bodily movement, the raise of the temperature of some object) and those describing the maintenance of some state (i.e., remaining seated), but excluding things such as time and spatiotemporal regions (and their parts), even though such things are classified as occurrents in some ontologies – e.g., the Basic Formal Ontology (Grenon & Smith, 2004).
The primary objective here is not to fully describe the ontologies nor to assess them with respect to their coverage of occurrents. Rather, we intend to gather interesting ideas about the nature of occurrents and about what should be considered when modeling such type of thing. Therefore, we do not intend to compare ontologies end to end, nor to explain how to use each of the ontologies to represent some event, but rather to present the general approach proposed by each of them concerning four main facets of occurrents: the definition of occurrent, continuant participation, mereology, causation. Moreover, we do not take into account the formalization used on the construction of the ontologies, except when it is useful to reveal some important facet of the notion of occurrent brought by the respective ontology. This means we are interested, for example, in which types of event an ontology supports, regardless if they are represented as a taxonomy or using some sort of “type” attribute, or in whether participants may play roles in events, no matter if these roles are represented as categories the individuals may fall into or as attributes they may present. Finally, in this paper we refrain from making further philosophical discussions about the nature of occurrents beyond what is needed to describe what is proposed by the analyzed ontologies – the reader interested in such debate should refer to Casati & Varzi (2015).
This survey considers 6 general upper ontologies and 5 ontologies that only cover occurrents. The upper ontologies are YAMATO (which stands for “Yet Another More Advanced Top-Level Ontology”) (Mizoguchi, 2010), the Unified Foundational Ontology (UFO; Guizzardi, 2005), the Descriptive Ontology for Linguistic and Cognitive Engineering (DOLCE; Gangemi et al., 2002), the Suggested Upper Merged Ontology (SUMO; Niles & Pease, 2001), the General Foundational Ontology (GFO; Herre, 2010), and the Basic Formal Ontology (BFO; Grenon & Smith, 2004). The other ontologies are the Event-Model-F (Scherp, Franz, Saathoff & Staab, 2012), the Simple Event Model (SEM; van Hage et al., 2011), the Event and Situation Ontology (ESO; Segers et al., 2015), the ontology underlying the Versatile Event Logic (hereafter referred to as “VEL”) (Bennett & Galton, 2001), and the ontology presented in Kaneiwa, Iwazume & Fukuda (2007) – hereafter referred to as “KAN”. The list includes both philosophically well-founded ontologies (e.g., BFO, DOLCE, UFO, GFO, YAMATO) as well as some more pragmatically-oriented ones (e.g., SUMO, SEM, ESO).
For each of the considered ontologies, we will first analyze some general aspects, e.g., the conceptual priority between occurrents and continuants and whether or not the ontology presents a taxonomy for occurrents. Then, we will analyze the four main facets of occurrent (i.e., definition, participation, mereology, and causation). The aspects related to each of the facets are divided into three groups: (i) the Pervasive Aspects (i.e., that are shared by most of the ontologies, at least implicitly, and do not conflict with commitments of any of the ontologies), (ii) the Complementary Aspects (i.e., that are present in a minority of the ontologies, but that are seemingly compatible with the commitments of the other ontologies), (iii) and the Conflicting Aspects (i.e., that are not compatible with commitments of some other ontology). Besides those main facets, we point out other interesting issues covered by some of the ontologies. In the sections that deal with each of the facets, the ontologies are generally presented starting from the one with the least commitments with respect to the facet in hand, proceeding to the ontologies with progressively more commitments or with more refined approaches regarding the facet in analysis. Finally, we will comment about some of the criteria used to branch the taxonomic trees of the ontologies – i.e., criteria used to subdivide a concept in its specializations – pointing out to which properties (or other aspects) each of these criteria corresponds.
This paper is structured in 10 sections, including this introduction. Section 2 makes clear some of the terminology used in this paper and briefly presents the inspected ontologies (the objectives posed in their construction, their scope, and other relevant particularities). The following four sections deal with the four considered facets of occurrents: Section 3 presents the definitions of occurrent, Section 4 analyzes participation, Section 5 discusses partitioning of occurrents, and Section 6 covers causation – with a summary of the findings at the end of each of these sections, indicating the pervasive, complementary, and conflicting aspects regarding the facet discussed in the section. Section 7 brings other interesting aspects contemplated by some of the ontologies. Section 8 recollects some event classification criteria employed on the ontologies. Section 9 shortly discusses the findings and presents two tables – one summarizing the aspects related to each facet, the other summarizing the occurrent classification criteria. Section 10 poses our final considerations.
Employed terminology and considered ontologies
The ontologies considered in this survey greatly vary in the terms used to refer to some notions (e.g., the most general category of things that happen in time is referred to as “occurrent”, “perdurant”, “process”, or “event” in different ontologies). Therefore, we will define here a set of terms to refer to common notions needed to make sense of the domain of occurrents. These will be preferred to refer to such notions instead of the specific terms used in each of the analyzed ontologies, except when the specific term is useful to highlight some relevant nuance or to make the text easier to follow (and, in such cases, we will make clear what we are referring to). Along with that, we will briefly present each of the considered ontologies, in order to provide the context in which the ideas they proposed have emerged.
Employed terminology
In this paper, we will employ the term occurrent to refer to the most general type of thing that happens in time (usually referred to as “perdurant”, “occurrence”, or simply “event”). Analogously, we will employ the term continuant to refer to the most basic type of thing that exists in time and endure change (which is called “endurant” or even “object” in some ontologies). We will use prolonged occurrent to refer to occurrents that happen over a non-instantaneous interval (e.g., the writing of a book) and instantaneous occurrent to indicate occurrents that are restricted to a single instant of time or that are temporally non-divisible at the considered granularity level (e.g., the ignition of a fire). Temporal region and time instant/point will be preferred to refer to the position or distribution of things in time.
The term object will be used to refer to existentially independent continuants (e.g., a person, a rock), while quality will be used to represent the intrinsic features presented by independent continuants at some point in time – both those representing valued attributes (i.e., color, size) or representing conditions, dispositions and the like (i.e., a fever, magnetic nature). Spatial region and spatial point will be employed to locate things on the 3-dimensional space, with spatiotemporal region and spatiotemporal point being their analogous in 4-dimensional space.
The term situation will refer to some configuration of a considered portion of the world at some spatial region and time point or interval (i.e., the set of objects that are present in such spatiotemporal region, along with the properties they present and relationships established among these objects, e.g., a prepared patient and a surgical team on an operating room right before a surgery). The term role will be used to refer to the common sense notion of role, as those contingent types that an instance may assume in some context, such as relational roles (e.g., student), occurrent-bounded roles (i.e., roles that an entity play with respect to an occurrent, e.g., agent), or roles of some other nature. The term relation will be employed to refer to something that holds between entities, connecting them in some sense (for which sometimes it is used the terms “relationship” or “object property”), and the term relationship will be regarded in a broader sense (e.g., “the relationship between these two notions”). We will resort to the term concept to broadly refer to the notion of a set of properties which form an ideal model that can be concretely exemplified in the world (for which sometimes it is used terms like “universal” or “class”). Complementarily, we will employ the term instance to refer to concrete exemplifications of concepts (for which sometimes it is used terms like “particular” or “individual”). Additionally, we will use the terms type and subtype to emphasize the idea of taxonomic division (e.g. “types of occurrents” would mean the various specializations of the concept of “occurrent”).
Considered Ontologies
There is a considerable number of ontologies that deal, at least in part, with the issue of occurrents. Then, bearing in mind that the focus of the paper is not assessing different ontologies, but rather bringing together remarkable ideas about the representation of occurrents (which may help to solve practical modeling questions, or face questions on the very nature of occurrents), we made our best efforts to have a sample of ontologies that is representative, but still small enough to allow treating the noteworthy aspects of occurrents in detail. Thus, since we are interested in the most general aspects involving occurrents, we avoided considering domain ontologies of any kind (e.g., CIDOC1
). As we are looking for somewhat reasoned aspects, we also avoided ontologies that do not have some paper or technical report in order to provide a glance on the reasons according to which its aspects were proposed (e.g., gist2). Moreover, since the focus is reuniting ideas, not evaluating specific ontologies, and in order to keep the sample on a manageable size, we did not include ontologies that only deal with aspects of occurrents that were already covered in some way by other ontologies already considered (e.g., Event Ontology,3 LODE4). Then, in order to provide some generality, the sample includes both philosophically well-founded efforts as well as more pragmatically-oriented ones, being composed of 6 very important general upper/foundational ontologies and some other ontologies that were specifically designed to deal with occurrents. Each of such works is presented as follows.The
The
The
The
The ontology presented in Kaneiwa, Iwazume & Fukuda (2007) – which we will refer to as “
The
The
The
The
The
The definitions of occurrent brought by the ontologies included in this review range from simply saying that occurrent is something that happens in time (an idea virtually shared by all of the considered ontologies), to definitions that emphasize the nature of its relationship with time (e.g., having temporal parts, being partially present at any time it is present), that focus on its effect on the world (e.g., change of something’s qualities, transition between states), or that are founded on its very relationship with continuants (e.g., manifestations of qualities of objects). We discuss such facets of the definition of occurrent on the following sections.
Definitions of occurrent according to different ontologies
In view of discussing the definitions of occurrent proposed by each ontology, we may consider the very loose and hopefully uncontroversial (at least with respect to the ontologies reviewed in this paper) definition of occurrent earlier presented (i.e., anything that happens in time) and, starting from that, go on adding details gathered from the definitions proposed by each ontology.
Indeed, this definition is very close to that proposed by
In DOLCE, the concept of occurrent comprises those of state (i.e., occurrent that is both cumulative7
A type of occurrent for which the mereological sum of its instances is still an instance of such type.
A type of occurrent for which the temporal parts of its instances are also instances of such type.
According to the
In a sort of follow up work on the conceptual basis of UFO (Guarino & Guizzardi, 2016) (which, for the best of our knowledge, is not yet included on the ontology), the authors propose that the prior concept event would be solely a subtype of occurrent, introducing the concept of scene (i.e., whatever happens in a suitably restricted spatiotemporal region – the maximal occurrent on such region) as the complementary type. Following from that, they enrich the definition of event as whatever happens to a selected set of individual qualities of objects in a particular spatiotemporal region. This set of qualities would be the focus of the event, with different foci yielding different events, so that it would work both as a principle of individuation and an identity criteria. Therefore, scenes could be roughly defined as “occurrents without focus” and events as “occurrents with focus”, with events emerging from scenes through a process of focusing on a selected set of individual qualities of one or more objects. Finally, according to UFO, occurrents cannot change in time. Such as in DOLCE, if an occurrent presents some characteristic at a time and presents a different characteristic at another instant, it is just two of its temporal parts presenting different properties.
For
In YAMATO, there is an important distinction between two types of occurrent: processes and events. A process is regarded as ongoing stuff, having no whole and no temporal parts (e.g., the process of “running”). On the other hand, an event is a unitary whole with respect to time, representing a distinct individual episode with a definite beginning and end (e.g., a run). In other words, whereas processes are non-discrete, dissective (i.e., any part of a process is itself a process of the same kind as the whole), having no definite extension, events are discrete, non-dissective and with a definite extension. In addition to that, the relationship between events and processes is analogous to that between objects and matter, so that events are constituted by processes as their material, being “chunks” of processes delineated by boundary events (e.g., start, end) – except for those very events that act as boundaries. Following form that, analogously to the case of objects, that can vary in complexity (e.g., a piece of wood vs. a table), events can vary from simple process chunks (e.g., a hike) to complex arrangements of qualitatively distinct process-chunks ordered in a certain way to qualify as a given type of event (e.g., an exploration journey).
Finally, in YAMATO there are certain types of occurrent which may change in time – namely, processes. In this ontology, a process is not a whole and have no parts and seems to be treated as a pattern of occurrence that can be framed in a “time window”. Such time window represents a suitable time resolution corresponding to a time length in which one can observe the whole pattern unfolding (e.g., for a “walking” process, the time window would be such that it would be possible to observe full steps, not just the isolated movements of a single leg). Thus, as such time window progress in time, we observe different stages of the same process (in a way analogous to that we would observe different time slices of a continuant), which may present differences in the occurrence pattern in relation to earlier stages (e.g., the speed of walking may increase in the next stage).
Ontical connectedness refers to the fact that presentials manifested at some time instant have their origin on earlier boundaries, in a sense similar to that of “such-as-to-have-come-forth-from” pointed out in Grenon & Smith (2004).
Causal connectedness holds when later boundaries occur as consequences of their predecessor boundaries.
GFO presents the concept of material process as a process whose boundaries are “material structures” (i.e., temporal slices of objects/independent continuants), in opposition to the concept of quality process (i.e., whose boundaries are simply aggregate of qualities). It also brings the concepts of change (i.e., transition between two processual boundaries that present contradictory properties), discrete/instantaneous change (i.e., a change in which the contradictory processual boundaries occur one immediately after the other), and continuous change (i.e., in which the contradictory processual boundaries are apart in time). Following from that, it presents the concepts of continuous process (i.e., process with no discrete changes within it) and discrete process (i.e., having discrete changes). As a last note, GFO also presents the concepts of event (i.e., a processual boundary that presents some property that is not verified either on the immediately previous or the immediately following processual boundary) and action (i.e., corresponding to the idea of an occurrent that is caused by an agent).
Summarizing this section, we can divide the aspects included in the definitions provided by each ontology in the three groups proposed in the introduction: (i) Pervasive Aspects, (ii) Complementary Aspects, and (iii) Conflicting Aspects.
Concerning (i), all the analyzed ontologies commit to a notion of occurrent that is disjoint from that of continuant,11
Even though, with the concept of process in YAMATO (which in some respects is more similar to continuants than to occurrents, e.g., being wholly present whenever present and enduring changes), such distinction is not so crispy.
With respect to the complementary aspects (ii), most of them are related to the inclusion of specific subtypes of occurrents. For example, there are occurrents having agents or not, having or not material structure, and having or not focus (which is noticeable both in the division between scene and event in UFO, as well as in the division between events that act upon a diffuse environment or upon specific objects, as proposed in KAN). We also have instantaneous or prolonged occurrents, counterfactual occurrents (such as the fictional ones in SEM or hypothetical in ESO) and those that represent an obtained result (as the event in GFO). The other complementary aspects represent modeling ideas that might help to enrich the ontological analysis of occurrents. Among them, we can mention the views of occurrents as transitions between situations, the coherence of processes in GFO (i.e., the causal and ontical connections between temporal neighbor slices of a participant), and the constitution of structured occurrents by “amorphous” ones (i.e., the relation between events and processes in YAMATO). We may also include the nature of occurrents as manifestation of individualized qualities (which is explicitly stated in UFO and BFO, but that may have some traces of it on the definition of purely quality processes in GFO), even though, in some sense, it may contradict the occurrent priority in GFO, since occurrents as manifestations of qualities are dependent on such qualities and then indirectly dependent on their continuant bearers.
To conclude the section, there are the conflicting aspects (iii), which are not compatible among themselves and could not (or at least hardly) coexist on the same ontology. Thus, they roughly correspond to different views on reality as a set of excluding options from which the modeler should choose according to her/his own view and the needs of the domain in hand. Among them, we can highlight the possibility of change for occurrents12
For further thoughts on this issue, the reader may refer to Guarino (2017).
An important aspect closely related to the very definition of occurrent (Mizoguchi, 2010) is the notion of participation, which is taken into account by all the ontologies considered. Participation is thought as the main relation between continuants and occurrents, intuitively being the involvement of a continuant in an occurrent, as if continuants in some sense “lived” in time by participating in some perdurant (Gangemi et al., 2002). In ontologies that distinguish continuants from occurrents (which is the case in all the ontologies considered), participation cannot be equated to parthood, since only occurrents can be part of occurrents (Grenon & Smith, 2004). Thus, it requires a special treatment, that is discussed in this section. Along with that, this section also covers the notion of occurrent-bounded roles (i.e., roles that continuants may play with respect to occurrents, due to the way it participates in it) and some alternative relations between continuants and occurrents brought by some of the ontologies.
Participation according to different ontologies
On the other hand,
YAMATO makes a distinction between the notions of external and internal process, defining that objects enact their external processes and are “supported by” their internal processes (e.g., a windmill converts energy into useful work as its external process and has the rotation of its vanes as an internal process). Based on this distinction, the ontology poses additional constraints over the notion of participation. For example, if an object is enacting a process, no proper part of the object is enacting that process (e.g., a human walks, but her/his legs do not – although the legs certainly enact some internal process that contributes to the overall walking). Following from that, processes are enacted by a single individual at any one time. Therefore, if a process seems to be enacted by several different individuals, it is in fact enacted by a composite individual which has those individuals as parts – each of them enacting its own partial process that contributes to the overall process. For example, if two people are pushing a heavy table, the manipulation of the table by each of these people are internal processes of the composite formed by the two people, whose external process is pushing the whole table. Moreover, this constraint is even proposed as unity and identity criterion for objects (i.e., object is a unity which is what enacts its external process, being an interface between its internal and external processes).
Still based on the division between internal and external processes, YAMATO also acknowledges, in a somewhat peculiar way, the possibility that an object plays a role in a process. Any object involves external processes that the object enacts and some internal processes that contribute to the enactment of such external process (e.g., a vehicle has “moving” as its external process and “providing motion power” as one of its internal processes). Each internal process of the object is specified by a role, which forms part of the definition of the object (e.g., “providing motion power” is associated with the role “power source”). Each internal role is filled by a role-holder object which enacts the process specified by the role. The role-holder may be internal or external to the object itself – if internal, it is a component; if external, an auxiliary object (e.g., for a car, the role-holder of “power source” is internal – the engine – whereas for a bicycle it is external – the person riding the bike). With that, a role-holder participates in the external process of the main object playing its internal role. In other words, it plays an internal role in the context of the external process. Finally, the processes which determine the nature of an object (i.e., its external processes) are causally dependent on the internal processes of the object. Due to this causal dependence, the object could not exist as it is without its internal process (i.e., internal processes sustains the object by giving rise to the external processes that characterize it as such). Therefore, according to YAMATO, besides enacting external processes and events, objects also relate to occurrents by being sustained by their internal processes and events.
Moreover, SEM explicitly accepts that instances of its four core classes (i.e., actor, event, place, and time) may play a role with respect to occurrents. This opens the possibility that not only continuants but also places, times and occurrents can play occurrent-bounded roles. Finally, neither ESO nor SEM provide further specifications about the semantics of the way an actor participates (e.g., in an active or passive way) or the possible types of roles, which must come from some domain ontology.
In
In
There are several forms of participation in
In GFO, presentials may refer to time slices of material structures, amounts of substrate, and qualities. Thus, since GFO states participation as a relation between any presential and a process, it commits to the participation of both independent and dependent continuants – what is also observable on the definition quality process (i.e., process with any of its boundaries presenting an aggregate of qualities). As a last remark, the paper that presents GFO also mentions the participation of processes in other processes by means of the particproc relation. According to it, a process x would participate in a process y if x is a layer-part of y – which seems to make this relation close to that of parthood (that is discussed in Section 5).
In
In
Based on such dimensions, BFO brings some specific forms of participation. The first one is perpetration, i.e., direct and agentive participation of the object that perpetrates an action, which can be further specialized in initiation, i.e., initiating an occurrent, perpetuation, i.e., sustaining an occurrent, and termination, i.e., terminating an occurrent. There is also influence, i.e., having an effect on an occurrent, that is divided into facilitation, i.e., having a positive effect on the unfolding of an occurrent, and hindrance, i.e., having a negative effect on the unfolding of an occurrent. Finally, there is mediation, i.e., having an indirect role in the unfolding of an occurrent which relates other participants, and patiency, i.e., being acted upon by an occurrent.
Besides the relation of participation (between objects and occurrents), BFO also offers the converse relation of involvement (e.g., as people participate in meetings, so meetings involve people), that somewhat represents the effects of an occurrent over its participants. An involvement may come in different forms, such as creation, i.e., bringing a participant into existence, sustaining in being, i.e., sustains a participant in being, degradation, i.e., having a negative effect over a participant, destruction, i.e., put a participant out of existence, demarcation, i.e., creating boundaries for a participant, and blurring, i.e., destroying the boundaries of a participant. Finally, although dependent continuants cannot participate in occurrents and neither occurrents can involve dependent continuants, BFO offers the relations of realization (which is analogous to participation) and affecting (which is analogous to involvement), both of which having some specializations.
Summarizing the ideas
As in the previous sections, we can also divide the aspects related to participation in the three groups proposed in the introduction – Pervasive Aspects, Complementary Aspects, and Conflicting Aspects.
In light of what was considered so far in this section, we can observe that all the considered ontologies commit to some notion of participation of continuants in occurrents. Moreover, all of them commit to the idea that participants may play different roles in the context of the occurrent in which they participate (except for VEL which does not explicitly commit to the idea of occurrent-bounded roles, but neither conflicts with it). Thus, these sum up the pervasive aspects concerning participation.
Complementing this initial account, some ontologies go on further specifying these notions (summing up the complementary aspects of participation). A participation may be classified by comparing its temporal extension with that of the occurrent or with that of the existence of the continuant (e.g., constant participation, temporary participation). Participations may also be classified by whether or not the participant participates on every non-temporal part of the occurrent (e.g., total participation), by the contribution of the participant to the occurrent (e.g., perpetration, initiation, perpetuation, termination), its effect over the unfolding of the occurrent (e.g., influence, facilitation, hindrance) among other criteria. Similarly, some types of occurrent-bounded roles are proposed (e.g., agent, patient, agent-patient, origin, destination, resource, instrument, result) based on a variety of aspects. Moreover, besides its typical purpose, the notion of participation may also have further implications on other issues. For example, it may be employed as criterion for occurrent partitioning, giving rise to specific types of occurrent, such as participation in UFO, and processual role in GFO, or as unity criterion for objects, such as in YAMATO. Additionally, we can observe that there are other possible relations between continuants and occurrents (e.g., involvement, realization, affecting, being sustained by).
Nevertheless, in spite of this initial convergence, there are incompatible aspects in the different definitions of participation (in other words, the conflicting aspects regarding this facet). Some ontologies, e.g., YAMATO, define participation as an explicit requirement to the happening of occurrents, while others are not so strict about this aspect, e.g., SUMO, which states that processes may have object participants, though it is not clearly explained why it is the case or how it would happen. Additionally, there are two views regarding the sorts of continuants that can participate in occurrents. The most prevalent one restricts the possible participants to the category of existentially independent continuants, i.e., objects. This is the view adopted by, for example, BFO, SUMO, VEL, and YAMATO. However, there is also a broader view, to which GFO commits, that accepts that any type of continuant can participate in occurrents, including both existentially dependent and independent ones. Finally, some ontologies admit that not only continuants can participate in occurrents. This seems to be the case of GFO, which accepts processes to be participants of other processes through the relation of particproc (though it is not so clear how it would differ from a parthood relation). Another ontology that does not restrict participation to continuants is SEM, which accepts that not only continuants, but also places, times, and even other occurrents can play occurrent-bounded roles. Considering that playing an occurrent-bounded role implies participating in the occurrent, this definitely conflicts with the predominant view that only continuants can participate in occurrents.
Occurrent partitioning
This section is intended to cover the mereology of occurrents, presenting the part-whole relations involving occurrents, the nature of its parts and other related issues introduced by each of the ontologies considered in this article. Here we do not try to impose any major restriction on what would count as part of an occurrent, nor on the criteria that may be used to identify such parts, nor on the types of relation between parts and the whole. Thus, we adopt a very loose approach, including anything that would resemble a part-whole pattern, considering not only parts with respect to time, but also those with respect to space, with respect to participants, and so on, with the only constraint that both parts and wholes must be occurrents.
Partitioning according to different ontologies
Except for
The ontology underlying
In the
According to
For
In
Finally,
Summarizing the ideas
Once more, we divide the aspects related to mereology into Pervasive Aspects, Complementary Aspects, and Conflicting Aspects, as proposed in introduction. The idea that occurrents may have temporal parts is apparently undisputed among the considered ontologies since most of them explicitly commit to such idea and the only one that does not deal with this aspect (i.e., ESO) does not pose any objection to it either. Such parts would, then, be both temporally and spatially included within the composite occurrent. It also seems uncontested that the part-of relation is a partial order relation – though it is not agreed upon whether it is a strict partial order (as in UFO), or non-strict one (as in SUMO, which does not restrict parts of an occurrent to its proper parts). These would be the pervasive aspects regarding mereology.
Enriching the picture, we can identify several complementary aspects. Some ontologies bring additional descriptions of what would be a temporal part or how the partition of occurrents in temporal parts should be regarded. From that, we end up with the idea that temporal parts are the sum of all cotemporal parts of an occurrent within a time interval (seemingly encompassing the corresponding temporal portions of any other orthogonal part of the occurrent, e.g., spatial parts, participations). As well, we should consider the existence of indivisible parts of occurrents (atomistic mereology), the notion of a complex occurrent as that composed of two or more disjoint parts (minimum mereology), and that parts of an occurrent are related to its identity (extensional mereology).
Still, probably more remarkable than that, we have the complementary features that the ontologies provide for the analysis and representation of occurrents. Besides the partition of an occurrent according to time intervals, the ontologies provide diverse criteria according to which one can split occurrents, each of them issuing a different type of part. Following the very idea of partitioning occurrents into temporal parts, we can verify whether such parts are founded on a real discontinuity on the flow of the occurrent or on an arbitrary choice of a time instant, yielding, respectively, bona fide or fiat temporal parts. Analogously to the division according to time intervals, one can partition an occurrent in spatial parts if considering the spatial region where it occurs. Paying attention to how the parts of an occurrent contribute to its unfolding, we can decompose occurrents in their causal (or functional) parts. If we turn our focus to the participants of an occurrent, we can divide it into the portions that exclusively refer to each of its participants, issuing the notion of participations (from which we may derive the notion of spatial part, if considering that all spatial qualities of occurrents are given by their participants). We can also target individual qualities that are involved in an occurrent (instead of whole participants) to split it into parts, what is used to identify events within scenes, in the UFO sense, but that could be applied to different scenarios.
Along with the idea of composition, we are also acquainted with the possibility of some occurrents being constituted by occurrents of a slightly different nature, which is proposed in YAMATO with events being constituted by processes. Here we have a somewhat restrictive notion, whose application would involve accepting the existence of a type of occurrent that conflicts with most ontologies (i.e., processes, as patterns of occurrence that can change in time). Nevertheless, the idea that an occurrent can be regarded as an additional layer of arrangements and restrictions over a simpler sort of occurrent may be a powerful tool for the modeler in need of making sense of a domain rich in occurrents.
Shifting from the instance level to the type level, we find the notion of typicality of parts of occurrents. With that, we can describe a given type of occurrent in terms of the types of occurrent whose instances usually compose the instances of the type considered. The other way around, we can also describe a type of occurrent as gathering instances that are usually part of another type of occurrent. If we choose a stricter condition, stating that the instances of a given type are always part of instances of another type, we arrive on the notion of essential part for an occurrent type. Both notions of typical and essential parts are expressed by type-level relations. Complementing these type-level concerns, we are introduced to the possibility of further constraining the possible parts of instance, not only defining the type of occurrent into which the part must be classified, but also some specific time or space regions in which the part must be included or some spatiotemporal region through which the part of the occurrent must unfold.
As a last thought, the selected ontologies present some possibly conflicting views on partitioning. For example, depending on the view adopted, one can acknowledge the existence of instantaneous temporal parts of prolonged occurrents (such as in BFO) or must reject it, having only prolonged occurrents as parts of prolonged occurrents (as in GFO). We can also mention the view according to which it is possible to have an occurrent composed of parts that are not spatiotemporally connected (as in the case of aggregate of processes in BFO, that accepts instances such as the set of all wars during a period of time), which is not accepted (at least explicitly) by the other ontologies – specially by GFO view, that requires that the boundaries of an occurrent (i.e., the temporal slices of the continuants participating on the occurrent) are ontically and causally connected.
Causation
This section discusses another noteworthy aspect for the Ontology of occurrents: the relation between causes and effects (for which it will be used the terms causation and causality interchangeably). Again, we do not try to impose any restriction on what would qualify as causation nor on what could be considered as causal relata.
Causation according to different ontologies
The notion of causation is found in most of the ontologies analyzed, except for
In
In
Except for the concept of scene, in
Yet not providing a detailed treatment,
Summarizing the ideas
As pervasive aspects of causation, the considered ontologies seem to agree on a general notion of causation as a binary relation holding between occurrents that represent a cause and an effect, with causes happening before effects. Moreover, many of the ontologies appear to understand causation in a sense closer to that of counterfactual dependence (e.g., Event-Model-F, with causes as necessary conditions for effects). This provides an initial approach to include causation concerns on some model.
On top of this initial basic notion of causation, the different ontologies add further specificities and unveil diverse complementary aspects concerning the definition of this relation. It is accepted the existence of versions of this relation which hold between individual instances – so as to state that a particular occurrence causes another particular occurrence – and others that are established between types – defining that the occurrence of an instance of a given type would cause (or be caused by) the occurrence of an instance of another type. It is also proposed the notion of situation as something close to the idea of causal link, with a cause occurrent eventually bringing about a situation that triggers the occurrence of an effect occurrent. Along with the idea of causation as a binary relation, it is also present the view that each cause causes a single effect and that each effect is caused by a single cause. With that, a single cause would be represented either by the mereological sum of all the occurrents that makes up the single cause – or by the maximal situation that triggers an effect occurrent. Likewise, a single effect would be the mereological sum of all occurrents that are triggered by a single cause – or the maximal situation brought about by the single cause occurrent, encompassing all the changes it performed). Finishing the list, besides counterfactual dependence, causation can be also regarded as regularity and manipulability.
Causal relation between instances is generally regarded as transitive – although this is not necessarily the case for type-level causation. The ontologies also present some specializations of this relation, according to the temporal relation between causes and effects (as in KAN, yielding disjoint, continuous, overlapping and partial causal relations). Causal relation is also specialized according to the existence or not of intermediary occurrents between a cause and its final effect (as in UFO, which specifies the directly-causes relation). Besides that, it is also considered the possibility of specializing the relation of causation to domain-specific types of causation. Along with that, a justification according to some underlying causal theory is also considered as part of the description of a causal relation.
The ontologies also present some seemingly irreconcilable divergences (i.e., conflicting aspects). Each of the ontologies accepts (at least implicitly) the notion of causation between occurrents. However, whereas some ontologies explicitly restrict this relation to hold solely between occurrents, others (i.e., KAN, GFO) accept that continuants may also be causal relata. Furthermore, while most ontologies regard causation as relating only things of the same nature (i.e., occurrent to occurrent, continuant to continuant), KAN acknowledges any combination (i.e., including the possibilities of continuants causing occurrents and vice-versa). Another source of conflict is the direction of the relation. Again, all the ontologies accept the unidirectional causation (from cause to effect, with the former preceding the later). Nevertheless, while some ontologies only accept this kind of relation, others consider the possibility of bidirectional/simultaneous causation (with both occurrents causally contributing to each other). As a last divergent aspect, SUMO explicitly states that causal relation can be only established between separate individual occurrents (not between an occurrent and its parts), while YAMATO implicitly commits to some causal relation between an occurrent and its causal (or functional) parts.
Finally, the ontologies provide some cases in which the establishment of causal relations can be witnessed. The already mentioned link between an occurrent and its causal parts is one of such cases. Another one is related to the particular view of YAMATO on the nature of objects as interfaces between internal and external processes. Causation is also present in the notion of correlation (present in the correlation pattern in Event-Model-F and in the notion of pseudo-process in YAMATO). The last, and probably the most significant one, is the causal relation between the successive stages of an occurrent, be them either the temporal parts of the occurrent or the temporal slices of continuants exhibited at each time instant (as in GFO). In the case of GFO, it is also present on the inertial principle, according to which there must be some external cause for a state change (with state considered as a subtype of process).
Other aspects
The previous sections covered some arguably key facets of occurrents (definition, participation, partitioning, and causation) that are present in most of the ontologies considered. Nevertheless, these works also deal with other interesting issues that, although not widespread over the ontologies (and sometimes not even fully developed on the works that bring them), may be of interest for practitioners. This section shed some light on a few of such issues.
Observer-related issues
At first glance, anything related to the idea of observer seems to better fall under epistemological matters rather than ontological ones. It is certainly the case when we care about how we acquire knowledge and update our beliefs – or, to put it simply, when we are on the position of an observer trying to make sure that what we know is correct. However, we may think about it from an external standing point, as the dynamics involving such a thing as an observer and things such as ideas, views, or beliefs which the observer gets upon the observation of something else. Put this way, talking about the existence of such things, ontological considerations become appropriate.
In part, this issue emerges from the problem of reconciling conflicting views about any given single instance of occurrent – and some ontologies provide means to meet this need. For instance, as stated in van Hage et al. (2011), SEM is “meant to record different, possibly conflicting, descriptions of an event as co-existing facts”. For that purpose, SEM includes the notion of view as a property that only holds according to a certain authority, which may or may not participate in the event. Views can be used to constrain roles, that would apply for continuants, places, times, and even for other occurrents. Therefore, using views, one could represent, for example, different roles for participants (e.g., hero, villain) or occurrents (e.g., victory or defeat, improvement or worsening), according to different authorities. With a similar purpose, Event-Model-F offers the interpretation pattern to represent divergent interpretations over the same occurrent. In this pattern, an interpretation is a situation that includes an occurrent (the F:Interpretant) and another situation representing the context according to which the occurrent may be interpreted. This contextual situation can be of any of the types represented by the other patterns the model provides (e.g., with causality situation, one can represent an interpretation regarding the cause or effect of an occurrent; for interpretations with respect to the participants, one can use a participation situation).
On the other side, observer-related issues are also on the very selection of what we are talking about when referring to occurrents. This faintly, implicitly appears in KAN when distinguishing object events and object states (i.e., occurrents which have a recognizable main participant) from environment events and environment states (i.e., those described using solely location and time, without mentioning any participant). Rather than solely a matter of observation, it seems to reveal a more fundamental feature, that allows classifying occurrents into two types: those that allow someone to spot the main participant and those that do not. In other words, some events are thought of as having a focal point whereas others are more diffusely described. Further in this line of thought, a follow-up work related to UFO (Guarino & Guizzardi, 2016) explicitly contemplates the issue of focus for events. But, instead of considering focal participants, it explicitly brings the finer notion of focus as a collection of individual qualities (i.e., focal qualities) to whose behavior one pays attention when identifying and individuating occurrents as well as describing them. Then, focus is used to divide occurrents into those that present a focus (i.e., events) and those that do not (i.e., scenes).
In somewhat different ways, both ontologies acknowledge the role of a focal point (or its absence) on classifying occurrents, making a distinction between diffuse and focal occurrents. For modelers, it also gives some aid in how to recognizing occurrents (i.e., how the abstraction of an occurrent could be conducted): either by gathering all the happenings in some spatiotemporal region/environment or by paying attention to to some specific portion of it (delimited by selected focal qualities or a focal participant). Moreover, the finer notion of focus as a set of focal qualities can help to solve some problems concerning individuation of occurrents – for example, as demonstrated in Guarino & Guizzardi (2016), it can solve the problem of individuating the events that are happening when a sphere is simultaneously heating and spinning (Davidson, 1969).
Roles for occurrents
It seems well accepted the idea that continuants may play roles, that come in two main types: relational roles (i.e., acquired in virtue of participation in some relation, e.g., the president of a council) and occurrent-bounded roles (i.e., acquired in virtue of participating and behaving in certain way with respect to an occurrent, e.g., the agent of an occurrent). Both types seem to present a relational nature (i.e., something plays a role with respect to something else, that is either an occurrent or another continuant) and a contingent nature (e.g., the role holder can play the role for just a period during its life).
On the other hand, the idea of occurrents playing roles of any kind usually faces objections, mainly due to the contingent nature of such idea, that seems to conflict with the widespread idea that events cannot suffer change – so that, if it cannot change, how could it acquire and lose roles?
In spite of that, among the considered ontologies, there are suggestions about the idea of occurrents playing roles. In the paper presenting the Event-Model-F (Scherp, Franz, Saathoff & Staab, 2012) it is stated that “only events can play the roles of cause and effect”, which certainly has a relational nature (i.e., an occurrent only plays the role of cause in relation to some other occurrent – the effect). Additionally, in some sense not explicit in the ontology, the role of cause could also be considered contingent (since, in the example of cause and effect, an occurrent is not a cause until its effect occurs). Analogous reasoning can be developed with the roles presented on other patterns brought by the ontology (e.g., events play the role of components in relation to a composite solely when the composite event is completed, with all its parts; interpretants play such role in relation to an interpreter just during interpretation process). Furthermore, whereas the notion of role in Event-Model-F is implicit, SEM explicitly allows specifying that an occurrent plays some role in the scope of another occurrent, although there is no further discussion on why considering this possibility or how such role would exist, since it would apparently imply the participation of an occurrent in another occurrent, which seems an odd idea.
The possibility of occurrents playing roles is certainly useful, both for allowing the representation of cases otherwise not possible and for helping to make explicit on models the contingent and relational nature of some types of occurrent. Along with that, accepting this option suggests diverse ways to analyze the context in which an occurrent happens. It may lead the modeler to take a step backward to look for other entities whose relations are necessary to make clear the nature of some occurrent. Also, considering this possibility may lead the modeler to search for some other occurrent that may serve as context, in which scope the role is played.
Contextual occurrents
Some ontologies suggest that, besides being what happens with their participants, some occurrents may be regarded as frames for the happening of other occurrents. The simplest case is that of scenes in UFO, defined as everything happening in a suitably restricted spatiotemporal area and from which events emerge – so that we can say that any given event happens in the context of a scene. BFO presents the concept of setting (or processual context) as an occurrent which stands as a spatiotemporal surrounding environment (or four-dimensional site) for other occurrents, in which they can be located to occur (e.g., a clinical trial can be the processual context for the discovery of a new treatment). In a similar approach, GFO presents situoids, which are occurrents representing an integrated whole made up of other occurrents.
Weaker notions of contextual occurrents can be found in YAMATO and KAN. YAMATO presents the concept of situation as an occurrent representing a state of affairs or the interaction pattern among participants (and their external states). It is introduced as a state shared by many participants, in opposition to the concept of state (which implies that the state is possessed by a single entity). Thus, in some sense, a situation could be seen as the sum of other smaller situations and/or individual states, being the context in which such smaller occurrents happen. Environment states in KAN, representing a state being kept by no particular identifiable object, may be regarded in a similar way, as the context for the states of possibly unfocused objects participating in the occurrent.
There are also restricted versions of this notion, applied to single entities, called histories in BFO and GFO or life in DOLCE. Generally speaking, they represent the mereological sum of all occurrents exclusively dependent on the continuant of interest or those that take place on the spatiotemporal region occupied by the continuant. This way, it would make sense to say that some occurrent took place in the context of the history/life of a particular continuant.
This notion of contextual occurrents provides an additional way to look for occurrents. Besides that of processes (in the sense of “cyclic occurrents”, e.g., walking), events (as transitions between situations, e.g., the baking of bread) and states (as some coherence that is actively kept by participants, e.g., being seated), one can take a step backwards and look at the big picture, identifying complex occurrents that frame others. Such a view would help to make explicit some diffuse, but yet relevant, occurrents (e.g., how could the Cold War occurrent be considered without the notion of contextual occurrent?). Furthermore, it may also be useful when it is necessary to capture faint or diffuse connections between occurrents, that would not be possible to depict using more direct relations, such as those of causation, correlation, or composition (e.g., all the sales that happen in a trade fair).
Occurrent classification criteria
Up to here, we discussed a variety of relevant aspects for identifying and representing occurrents. Nevertheless, one of the arguably prior reasons for using an ontology to model occurrents is being able to classify their instances according to the categories of the ontology. With that in mind, this section discusses those aspects that are used to branch the taxonomy of the considered ontologies – hereafter called occurrent classification criteria.
Occurrent classification criteria may appear in various types – e.g., it may be a quality that, depending on the value it presents, makes the occurrent instance falling into a different category (e.g., duration), or it may relate to the number of participants involved. It is important to note that the list here presented is not complete, but rather includes aspects that are more clearly recognizable. It also tends to privilege those aspects that virtually form a disjoint and complete partition (e.g., instantaneous vs. prolonged occurrents) over those that represent different ways to consider a concept, that are not necessarily disjoint (e.g., the criteria used to divide the taxonomy into a branch for simple/relational occurrents and a separated branch for atomic/complex ones, since both divisions virtually form partitions of the same set). Finally, the discussed criteria include both those explicitly stated (e.g., cumulativity, in DOLCE) and those inferred from the definitions of sibling concepts considered to be distinguished by the criteria (e.g., agentivity, when the main difference between two sibling concepts is the participation of some agent).
Concerning presentation, the selected occurrent classification criteria are gathered according to the more general aspect/facet of occurrents they are related to: participants, part-whole structure, temporal issues, dynamics, and focus/observer issues.
Participant-related criteria
The most numerous occurrent classification criteria are those related to the participants of the occurrent. The simplest criteria is the
Matters related to the agent role also serve as occurrent classification criteria.
Structural concerns are also taken into account.
What happens with the participants is also considered. YAMATO distinguishes action_behavior from motion, which seems to raise
Part-whole-related criteria
Part-whole issues are also a rich source of occurrent classification criteria.
Time-related criteria
Regarding temporal aspects,
Dynamics-related criteria
Regarding dynamics (here understood as related to whether occurrent is static or dynamic, and if dynamic, in which extent), the criteria of
Focus-related criteria
Considering focal and observer issues, we have
Discussion
Summarizing the findings, Table 1 gather the aspects of occurrents we have collected and analyzed concerning the four considered facets of occurrents (i.e., definition of occurrent, participation, mereology, and causation), dividing them into the three groups described in the introduction (i.e., Pervasive Aspects, Complementary Aspects, Conflicting Aspects). Along with that, Table 2 summarizes the occurrent classification criteria discussed in this paper.
Summary of findings about occurrents
Summary of findings about occurrents
Summary of occurrent classification criteria
Starting from that, we can think about the findings in two distinct ways. From one side, we can discuss what does it mean for an aspect to be in one of the three groups of aspects described in the introduction or to be used as a classification criterion – and how the modeler should regard them when facing the problem of modeling occurrents. From another, more pragmatic point of view, we can analyze the impact of the identified aspects on the process of analyzing and modeling occurrents (either when developing an ontology or when choosing an existent one to be employed as guidance).
Pervasive aspects refer to reasonably settled matters – apparently found on solid philosophical arguments and compliant with ordinary experience. Thus, they can be considered a basic common view about occurrents to which the considered ontologies commit. Not all of them are mandatory properties of occurrents, but rather properties that every (or at least most) of the ontologies explicitly accept. Likewise, they are non-exclusive, so that it is possible for other ontologies to accept additional alternative views concerning such aspects (e.g., having temporal parts does not exclude the possibility of having parts according to some orthogonal criteria). Considering that, the sum of such aspects constitutes a description of occurrent that may qualify as a basic, independently endorsed definition, which should not be overlooked and that apparently yields an initial picture for the treatment of occurrents (e.g., for identification, representation, reasoning).
Complementary aspects can be regarded as additional interesting ideas that researchers developed as a result of contemplating diverse domains or in response to different modeling needs. As minority views, they might represent less prevalent aspects or ideas that were more recently brought to debate – which may offer opportunities to consider what would be the implications if such a view were adopted in a greater variety of cases. Therefore, not including them on models may not cause major problems. On the other hand, since they do not conflict with commitments of the other ontologies (especially the pervasive aspects), they can be regarded as optional tools to be freely employed to enrich the model according to the particularities of the targeted domain or task.
In addition, these aspects seem to refer to things we only perceive when getting a closer look at the occurrent, as opposed to pervasive aspects, that are apparently immediately recognizable. For example, it is clear that occurrents happen in time, but conceiving them as quality manifestations requires further analysis (e.g., we must first identify the participants in order to determine which of their qualities are being manifested). It is also relatively straightforward to identify the participants of an occurrent (since we arguably only perceive it by observing its effects over the participants), but determining their roles demands more attention. Likewise, temporal parts are evident, which is not the case for the orthogonal divisions (e.g., the various participations that compose an occurrent are frequently intricately related to each other, being hard to individuate as significant single occurrents). Still, general causation between two occurrents seems intuitive, but it requires further thought to recognize additional characteristics of this notion (e.g., transitivity, situations as causal links between occurrents).
Conflicting aspects represent irreconcilable options, yielding modeling decisions that practitioners must take when analyzing a domain. This group encompasses both acknowledged open issues (e.g., the ontological priority between continuants and occurrents) and proposals of divergent paradigms (e.g., occurrents playing occurrent-bounded roles). Whilst some of them sound as minor divergences (e.g., possibility of instantaneous parts), some others relate to deeper problems, with far-reaching implications, leading to relevant differences on the models of specific domains (e.g., ontological priority, occurrents undergoing changes or playing roles w.r.t. other occurrents, bidirectional causation, continuants as causation relata).
Regarding occurrent classification criteria, it would probably not be too far-fetched to say that they probably help to uncover what we first notice when we think about occurrents. Therefore, having an instance of occurrent, it would be the case that we first classify it into the suitable type to later complete its description with additional details. Thinking this way, we may entertain an idea about what catches our attention the most on occurrents (always keeping in mind the limitations of the sample of considered ontologies). Observing the general facet/aspect to which the criteria are related, it is noticeable that the majority of the criteria are related to participant and part-whole issues, which may suggest that these are the main targets when someone is trying to make sense of some occurrent. On the other hand, moving the attention to the specific criteria and how frequently each of them is used across the considered ontologies, the most recurrent classification criteria are those related to how may participants the occurrent involves, whether or not the occurrent is a deliberate action, how it can be partitioned, which is its duration, and in which extension it represents a change. In light of that, the aspects/facets on which these criteria are based may configure, along with the pervasive aspects, an initial target for the task of analyzing occurrents.
Impacts over the analysis and modeling of occurrents
Analogously to a painter who has more options in his color palette to better depict the scene s/he observes, having a more extensive list of aspects to bear in mind when analyzing/modeling occurrents may allow the modeler to make more precise representations. Knowing about the diverse possible aspects under which to consider an occurrent, the modeler can choose the right combination to reveal and make explicit the relevant details of the domain, making it easier to convey the conceptualization that is already shared by the community.
The sum of the pervasive aspects gives us a rich definition of occurrents: occurrents are things that are not continuants, that happen in time accumulating temporal parts, that are associated with time intervals, that have continuants as participants (which may play different occurrent-bounded roles), that may represent changes or maintenance of states, and that are the relata of causal relations. This definition allows differentiating occurrents from continuants (which may seem somewhat obvious, but that may not be immediately accessible for the modeler since this is not a native distinction in most modeling languages). On top of that, the modeler analyzing an occurrent is led to ask when it has happened, which objects participated in it (and what were their contributions), and if it can be broken into significant temporal parts – ending up with a meaningful picture of the occurrent in hand. It would raise similar questions for a modeler trying to define a concept representing a type of occurrent from the domain in hand (e.g., how long is the interval typically associated with instances of such type, which kinds of participant it has, which are the relevant parts).
Moreover, acknowledging that occurrents can represent both change and maintenance of states broadens the scope of occurrent identification. Whereas recognizing an occurrent when it represents a change in the state of the world seems straightforward (i.e., it immediately follows from the perception of the difference from one instant to another), conceiving that there may be an underlying occurrent that is “keeping things as they are” requires a richer conceptual framework in order to come up with and investigate such possibility. Finally, viewing occurrents as relata of causal relations opens for the modeler the possibility of exploring the causation chain. Instead of just attributing the happening of some occurrent to the involved participants, s/he is also led to verify which were the previous events that led to the occurrent in analysis, as well as the possible occurrents that may happen in consequence of the present occurrent.
The remaining complementary or conflicting aspects have a similar impact. Concerning mereology, the different kinds of parts (spatial, causal/functional, by participant, by focusing qualities) indicate the joints into which an occurrent may be meaningfully partitioned, beyond merely arbitrary temporal parts. In addition, accepting that an event may have temporarily-disconnected parts permits us to recognize certain temporally scattered occurrents as a single occurrent (e.g., a soccer competition that happens during a whole year, but that is composed of weekly matches) in an analogous way we identify spatially scattered occurrents (e.g., an art festival composed by simultaneous artistic interventions in different theaters).
Participation is in a comparable situation. Committing to different types of participation (e.g., with respect to temporal extension) enables defining different degrees of involvement of the participants. Additionally, committing to different specific types of occurrent-bounded roles (e.g., agent, result) permits distinguishing what is the contribution of each participant to the occurrent as a whole. A similar case can be made for alternative continuant-occurrent relations – e.g., realization, affecting, being sustained by.
Regarding the nature of the occurrents, complementary definitions may help to reveal the existence of underlying occurrents that would not be identified with a simpler view. For example, considering occurrents as manifestations of qualities of objects allows the individualization of certain superimposed events – happening at the same spatiotemporal region, involving the same objects – that otherwise could only be seen as a single occurrent (e.g., the problem of the sphere simultaneously heating and spinning, presented in Section 7.1).
Considering occurrents as transitions between situations13
For further thoughts about this idea of occurrents as transitions between world states, the reader may refer to the work by Benevides & Masolo (2014).
Some commitments may also lead to unexpected consequences, that can be further explored by the modeler. For example, admitting that observers (which are continuants) may have different views or interpretations about a single event seems to imply the existence of a different kind of relationship between continuants and occurrents, which is not that of participation or those closely related ones (e.g., involvement, realization, being sustained by). In fact, differently from such “participation-like” relations, this “observing-like” relationship may be even established between a continuant that exists at a time and an occurrent that had happened long before the continuant came into existence (e.g., when a historian is indirectly observing a historical event by means of documents).
This paper presented an extended review of ontology-based conceptual modeling of occurrents. It considered 11 different ontologies, including both ontologies that exclusively cover occurrents and others that also deal with continuants. It also included both ontologies with strong philosophical concerns and those driven by more pragmatic principles. The review analyzed four main facets of the notion of occurrent (definition of occurrent, participation, partitioning, and causation) and took a look on the most prominent occurrent classification criteria used to build the conceptual-taxonomies of the ontologies.
Instead of describing each ontology separately, we opted for structuring our analysis around the selected facets of occurrents, articulating and contrasting the views of multiple ontologies around each of the aspects. Additionally, we group the aspects according to how they relate to the other aspects and how they are distributed among the considered ontologies – i.e., pervasive, complementary, or conflicting aspects. With that, instead of summarizing individual works, we tried to unveil and emphasize milestones in the research on the Ontology of occurrents. It is possible to observe that the selected ontologies share both a considerable set of common aspects (which reveals an aparent agreement upon a basic view on occurrents) and a sizable set of conflicting ones (which points to open issues and divergent approaches). Nevertheless, the largest share is composed by complementary aspects, which are scattered among the diverse ontologies. The same occurs with the occurrent classification criteria, since they are numerous and none of the ontologies encompasses all of them. Thus, this review may provide a starting point for efforts aiming to harmonize those views in a single approach that may enrich the analysis and representation of occurrents.
Considering this work from a different perspective, we brought a collection of already proposed solutions for some of the most sensible issues on the conceptual analysis and modeling of occurrents. Thus, beyond a glance on what is going on in the research about occurrents, what is presented here is a set of tools that may be worth to have in the modeler’s toolbox. So, in practical terms, the reader can take away from this paper
A basic, widely accepted description of occurrents (i.e., the pervasive aspects).
A set of additional tools to further analyze/describe occurrents (i.e., the complementary aspects).
A set of irreconcilable modeling options that require attention (i.e., the conflicting aspects).
A set of criteria to help in occurrent analysis and classification, that might also be combined to generate a particular taxonomy (i.e., the occurrent classification criteria).
Nevertheless, although the review covers a relevant set of ontologies (including some of the most important and widely used upper ontologies), it is not a comprehensive review – in the sense that the number of ontologies included in the study is relatively small. So it cannot be regarded as a full picture of what has been done so far on the representation of occurrents. Such limitation should be taken into account while interpreting the findings here described – specially regarding the evidence of open issues or identifiable research tendencies.
Following efforts may be directed to broadening this review, including other works not covered here. Alongside that, a similar review may be carried out exclusively with works on Philosophy and Cognitive Psychology in order to identify foundations and new insights for further developments on the conceptual modeling of occurrents.
Footnotes
Acknowledgements
This work was partially supported by the Brazilian Council of Research (CNPq) and by the Brazilian Coordination for the Improvement of Higher Education Personel (CAPES).
