Medieval Philosophy

by Peter S. Adamson

Antique Christians were convinced that they could find evidence for God’s Trinitarian nature in the Bible. Augustine’s On the Trinity devotes several books to the scriptural basis of the doctrine, which is, of course, central to Christian theology. But as Augustine goes on to show in his On the Trinity, the conceptual tools of philosophy can help us see how it is possible for one and the same substance to be three Persons.1 The core of the Trinitarian dogma is the claim that the three divine Persons differ from one another, while being the same God. But what does this mean? A first thought might be that each thing is the same as itself, and different from any other thing. This makes the Trinitarian doctrine look problematic. But further reflection will show that there are many ways of being the same or different. The Marx Brothers were four things, so not the same as each other in every respect; yet they were the same in respect of being human. Really, the problem of universals is just the difficulty of how things are able to share this sort of “sameness.” Then there is the problem of sameness over time. Am I the same as the boy who first watched those Reese’s Peanut Butter Cup commercials? That boy was different in