Abstract
In the field of information systems the term ``constraint'' is applied to statements of various kinds. Here we start from the analysis of a simple example to characterise the different kinds of constraints. It is shown that constraints may be necessary truths or deontic constraints. Moreover, deontic constraints are classified into three different types: deontic constraints about the world, deontic constraints about the representation of the world (self-completeness), and deontic constraints about the links between the world and its representation (validity and completeness). We describe a modal logical framework to define the different types of constraints, to characterise their violations, and to show how to repair their violations. Two different general forms of deontic constraints are considered, namely O(�→ψ) and �→O�, and it is shown that, except for deontic constraints about the world, the latter is more appropriate. Special issues related to the definition of quantifiers in the context of modal operators are also considered.