A Contingent Thing's Existence Must Be Really Different From Its Essence

Since it is not necessary but contingent, its existence must be really distinct from its essence. A third reason why the essence and existence of each of the things we know through experience must be distinct is that there cannot in principle be more than one such thing. There would have to be something by virtue which A and B are distinct things rather than one thing. But what could that be? The problem is that on analysis, it turns out that A and B could not be differentiated in either of these ways.