 We can trace the issue of whether existence is a property to a disagreement between the ancient Greek philosopher Aristotle and some of his medieval followers over the relationship between an individual’s essence and its existence. The debate requires some background. We begin with the distinction between accident and essence and that distinction’s relation to contingency and necessity. Some of a thing’s properties are contingent, in the sense that the thing might not have had them. I am writing right now, but I might have been out for a run instead. So working on a paper right now is one of my contingent properties. Contingent properties contrast with necessary properties. I am necessarily human, in the sense that it is impossible that I am a nonhuman. (It may be possible that it is not the case that I am human, insofar as it is possible that I never existed at all, in which case I would not have been, in that possibility, among the class of humans. But that is a different matter.) All contingent properties are accidents and all essences are necessary but, according to the Aristotelian, some necessary properties are accidents. A thing’s essential properties are inseparable from the bearer, not only in the sense that the property is necessarily had by that object (insofar as that object were to exist), but in the deeper sense that any adequate account of what that object is involves that property; they are part of any adequate definition of the thing or answer to the question ‘What is it?’. I am essentially a human and perhaps essentially the person I in fact am, if there are individual essences in addition to general essences. I am necessarily identical to something and necessarily such that 2+2=4, but these properties are among my accidents, as they are not part of any adequate account of what I am and what distinguishes me from others.  While this distinction between a thing’s necessary properties and its essential properties is controversial, there is no doubt that it was Aristotle’s view, shared by most of his medieval followers, and so informed the first historical occurrence of the debate about existence under discussion. Aristotle seems to have seen nothing more to existence than essence; there is not a space between an articulation of what a thing is and that thing’s existing.  Saint Thomas Aquinas, on the other hand, famously distinguished a thing’s essence from its existence. Aquinas argued something as follows, in chapter 4 of his On Being and Essence. One can have an understanding of what a man or a phoenix is without knowing whether it exists. So, existence is something in addition to essence.  In short, Aquinas argued that existence is a separate property as existence is not part of most objects’s natures and so those objects can be conceived or thought of separately from their existing.