Near the end of his book on holiness, John Webster writes that “a crucial aspect of holiness is an increase in concentration: the focusing of mind, will and affections on the holy God and his ways with us.” In this article, I briefly explore a few ideas in Thomas Aquinas’ exegetical work (chiefly in his commentaries) that constellate around the idea of holiness as concentration. I suggest that “concentration” offers a fruitful way to speak of a saintly analogy to divine simplicity. Not even saints are simple in the strict sense, but they can be sincere and pure, with their energies gathered together in a unified pursuit of a single, divine object. I then offer a reading of the famous Milanese garden scene in the Confessions, arguing that Augustine's conversion is paradigmatic of concentration, before concluding with an amendment and a commendation.