ABSTRACT
In his first years as a lecturer at the University of Paris, Thomas Aquinas wrote two short treatises at the request of his fellow friars, On the Principles of Nature and On Being and Essence. Likewise fundamental, On Being and Essence outlines Aquinas's metaphysical framework. What Aquinas insists on preserving from that Aristotelian project is the basic distinction between the being of substances and the being of all the other categories, accidental being. The clearest instances of substance, for Aquinas, are living things, because they have a unity and coherence that makes it highly plausible to describe them as a single thing, enduring through change, over time. Aquinas is, of course, aware that all material substances, particularly living ones, undergo constant and dramatic change. Aquinas's conception of essence rests on both a causal and a metaphysical claim. Aquinas's theory of essence is riot just the medieval ancestor to genetics or chemistry.
