You can say exactly what something would be — a role, a product, a company, a unicorn — in complete detail, down to every property it would have, and still not know whether there is one.

The specification and the existing are separate questions, and a complete answer to the first tells you nothing about the second. That gap is the distinction between essence and existence, and the scholastics built a metaphysics on it.

Existence is not one of the properties

The move that makes this more than an observation is negative.

You might think existence is just another item on the list — the unicorn has a horn, has four legs, and lacks existence. Aquinas says no, and for a good reason: a property is something an essence has, and having anything at all presupposes being there to have it. Existence cannot sit on the list because the list has no standing without it.

His formulation is that existence is the act of the essence. Not a feature it possesses but the actuality by which it is anything whatever. An essence without existence is not a thing lacking a property; it is not a thing at all.

This is why the vocabulary runs the way it does — act and potency, with essence standing to existence as potential stands to actuality.

Why anyone needed it

Avicenna put it to work first, and the payoff is an argument about dependence rather than about beginnings.

In everything you encounter, what it is and that it is come apart. Nothing in the nature of a horse accounts for there being horses. So the existence of any such thing is received from outside it, and the receiving is not a historical event but a present condition: the horse depends on something else now, not merely at some first moment.

That is the shape of the argument from contingency, which is treated on cosmological argument and on five ways rather than here. The relevant point for this page is what it requires at the end of the chain: something in which essence and existence do not come apart, whose nature is to be. What that commits you to is worked out under divine simplicity.

Kant's objection, and why it misses

Kant's famous line is that existence is not a real predicate — a hundred real thalers contain no more coins than a hundred possible ones, so adding "exists" to a concept adds nothing to it.

That is decisive against the ontological argument, which needs existence to be a perfection that a concept can contain. It is not an objection to the doctrine here, and the reason is worth seeing: Aquinas agrees. Existence is not a predicate on his account either. He is saying something stronger than Kant denies — not that existence is a property, but that it is what makes there be anything for properties to belong to.

Two claims that sound alike and are opposed. Kant says existence adds nothing to a concept; Aquinas says existence is not the kind of thing that could be in a concept at all.

Not Sartre's slogan

Existence precedes essence is the other famous sentence in this area and it is a different doctrine entirely.

Sartre is making a claim about human beings specifically: there is no human nature fixed in advance, so a person exists first and makes what they are through choosing. It is a thesis about freedom.

Aquinas's distinction applies to every created thing, horses and stones included, and says nothing about whether natures are fixed. The two use the same two words for unrelated purposes, and the coincidence has confused readers of both for a century.