Thomas Aquinas’s doctrine of the real distinction between essence and existence in all beings other than God has been the focus of much debate among Thomists. And this is how it should be, for all agree to its central importance in Aquinas’s metaphysics. Is the distinction a deduction we make from our knowledge of God’s essence, or an insight drawn from our experience? And if it is the latter, is this insight from multiplicity to unity based on the inevitable mental distinction we draw between the concept of essence and that of existence, or is it the fruit of a metaphysical penetration of the material things we meet within our world? Let us look first at the argument based on an intuition into God’s simplicity and a deduction from that intuition, and then turn to an examination of the arguments which move from the diversity of our experience to the simplicity of the source of that experience.
In the first argument, the claim is that we have a direct insight into being as being, into existence itself, that is, God. ‘Being (esse), insofar as it is being, cannot be diverse.’ If it differs, it must be by something other than it, that is, by particular essences. Is this the main-line argument for the real distinction? Many commentators (among them O’Brien and Fabro) have thought that it is. However, I believe they are mistaken. For the proof as it stands is incomplete, not in failing to offer a conclusion, but in failing to ground its major premise.