  Descartes’ reference to “essences” raises another important issue more directly related to the ontological argument. In claiming that necessary existence cannot be excluded from the essence of God, Descartes is drawing on the traditional medieval distinction between essence and existence. According to this distinction, one can say what something is (i.e. its essence), prior to knowing whether it exists. So, for example, one can define what a horse is — enumerating all of its essential properties — before knowing whether there are any horses in the world. The only exception to this distinction was thought to be God himself, whose essence just is to exist. It is easy to see how this traditional distinction could be exploited by a defender of the ontological argument. Existence is included in the essence of a supremely perfect being, but not in the essence of any finite thing. Thus it follows solely from the essence of the former that such a being actually exists. At times, Descartes appears to support this interpretation of the ontological argument. In the Fifth Replies, for example, he writes that “the existence of a triangle should not be compared with the existence of God, since the relation between existence and essence is manifestly quite different in the case of God from what it is in the case of the triangle. God is his own existence, but this is not true of the triangle” (AT 7:383; CSM 2:263). But Descartes’ complete view is subtler and more sophisticated than these remarks first suggest. Understanding this view requires a more careful investigation of the distinction between essence and existence as it appears in medieval sources. Although one often speaks of the “traditional” distinction, the exact nature of the relation between essence and existence in finite things was the subject of a fierce debate among medieval philosophers. Seeing where Descartes’ position fits within this debate will provide a deeper understanding of his version of the ontological argument.