What is the real distinction?

Thomas Aquinas, answered from the texts and cited to the page.

Thomas Aquinas answers

The question is whether, in a created substance, essence and existence are really distinct as two ontological components, or whether the distinction is only conceptual — a difference in how we think about the same thing, not a difference in the thing itself. Begin with what is not in dispute. In God alone, essence and existence are identical: what God is and that God is are one and the same.¹

Every other being receives existence; it does not simply be existence. That much all the schools grant. The quarrel is over what kind of distinction this receiving implies in the creature itself. The argument I press in the De Ente et Essentia — a short treatise on the nature of being and essence as such — runs as follows. The essence or quiddity of any being other than God can be conceived without including existence in its concept.²

A man can understand what human nature is and nonetheless doubt whether any man actually exists. Since existence is not contained in the notion of the essence, it must come from outside the essence and enter into composition with it. All substances except God are therefore composed of essence and existence.³ Now the hard question: does this argument establish a real composition — two genuine ontological principles in the creature — or only a conceptual one?

The same style of argument appears in the De Ente for the distinction between an individual and its specific nature, and there I hold that individual and nature are really identical, distinct only in thought.⁴ A reader who notices that parallel has a serious point: the argument form alone does not settle whether the distinction is real or conceptual.

What does settle it, or moves further toward settling it, is the causal consideration. If existence is not of the essence of a creature, it is adventitious to that essence — it accrues through another.⁵ To be caused by another does not belong to being as being, for otherwise every being would be caused by another, which is impossible since the series must terminate.⁶

That which is being of and by itself must therefore be uncaused. No caused being, then, is its own existence.⁷ This is not a merely logical point about how we conceive things; it is a point about causal dependence, which is a real relation. If the creature's existence were simply identical with its essence, there would be no account of why that existence requires a cause external to the essence — the essence would simply be its existence, as God is.

The parallel to participation confirms this. Existence is attributed essentially to God alone, since God's existence is subsistent and absolute.⁸ To every creature, existence is attributed by participation — no creature is its own existence but rather a thing that has existence.⁹ Whenever something belongs to a thing by participation, there must be something in that thing besides what is participated.

Hence in every creature, the creature that has existence is other than the existence it has — which is precisely what Boethius says in the De Hebdomadibus: in everything short of the first, what exists is other than its existence.¹⁰ Since everything outside a thing's essence is called an accident, the existence that answers the question Does it exist? is, in a qualified sense, accidental — not in the sense that it is a mere quality that might or might not inhere, but in the sense that it is not included in what the definition of the thing contains.¹¹

No creature's definition contains existing, since existence is not a genus or a specific difference.¹² Whether I have demonstrated a real distinction in the full metaphysical sense, or only a distinction that is conceptual but grounded in the causal structure of things, is a question the De Ente itself does not resolve with complete explicitness, and I acknowledge that.¹³

In other works I teach the real distinction plainly. Here the argument establishes at minimum that essence and existence in creatures are not simply the same, and the causal reasoning presses toward something stronger than a mere difference in our modes of conceiving.

Sources

in whatever reality essence and existence are identical, that reality is its own existence: in other words, it exists of itself, which self-existence is proper to God alone.
Of God and His Creatures, Summa Contra Gentiles abridged
the essence or quiddity of every other being can be conceived without including existence in its concept.
On Being and Essence, p. 8
since existence is not contained in the notion of its essence, it must come from outside the essence and enter into composition with it. All substances except God, therefore, are composed of essence and existence.
On Being and Essence, p. 8
In chapter three there is a similar proof for the distinction between an individual and its specific nature... But this does not establish a real distinction between the individual and its specific nature. St. Thomas, in fact, holds that these are really identical, and distinct only in our thought.
On Being and Essence, p. 8
But to every created reality existence accrues through another, otherwise it would not be a creature. Therefore of no created substance is it true to say that its existence is its substance.
Of God and His Creatures, Summa Contra Gentiles abridged
to be 'caused by another' does not belong to being, as being: otherwise every being would be caused by another, which is impossible.
Of God and His Creatures, Summa Contra Gentiles abridged
that existence which is being of itself and by itself, must be uncaused. No caused being therefore is its own existence.
Of God and His Creatures, Summa Contra Gentiles abridged
existing is attributed essentially to God alone, since God's existence is subsistent and absolute existence.
Quodlibetal Questions, pp. 318–319
existing is attributed to every creature by participation, since no creature is its own existence but, rather, a thing that has existence.
Quodlibetal Questions, pp. 318–319
in the case of every creature, the creature that has existence is other than the existence it has. That is just what Boethius says in his book On the Hebdomads: in the case of everything short of the first thing, what exists is other than its existence.
Quodlibetal Questions, pp. 318–319
since everything outside a thing's essence is called an accident, the existence relevant to the question 'Does it exist?' is an accident.
Quodlibetal Questions, pp. 318–319
no creature's definition contains existing, since that is not a genus or a specific difference.
Quodlibetal Questions, pp. 318–319
The present treatise does not answer these questions clearly. In other writings St. Thomas explicitly teaches that there is a real composition of essence and esse in created substances... But here he does not specify the kind of distinction established by this argument.
On Being and Essence, p. 8