What is essence and existence in philosophy?

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

Thomas Aquinas answers

Essence is what a thing is; existence is that it is. The distinction seems simple, and its consequences are not. Take any creature you like — a man, a phoenix, an angel. You can form a perfectly clear notion of what each is without thereby knowing whether any of them actually exists in reality.¹ The concept of humanity includes rationality, animality, the capacity for speech; it does not include the fact of existing.

Existence is not contained in the definition of a creature's essence, and because it is not contained there, it must come from outside the essence and enter into composition with it.² Every substance except God is therefore composed of essence and existence as two intrinsic principles. Now consider what follows if there were a being whose essence just is existence itself — not a being that has existence, but a being that is its own existing.

The notion of what such a being is would contain existence, and it alone would be incapable of not existing.³ This is why He who is — the name given in Exodus — is the most proper name of God: his substance is none other than his existence, and that is proper to God alone.⁴ The argument has another approach from the side of causality. All created causes, however different their proper effects, share one common effect: they cause being.

Fire makes a thing hot and thereby makes it to be; a builder causes a house to be. Since they share this effect, there must be some higher cause to which being properly belongs, as heat properly belongs to fire rather than to pepper or ginger, which merely produce heat by participation.⁵ That higher cause, whose proper effect is being itself, must therefore have being as its very essence or nature.

In everything else, existence accrues through another — which is precisely what it means to be a creature — and so no created substance is its own existence.⁶ I should be honest about what remains genuinely disputed among those who have worked through this carefully. The argument as I gave it in De Ente et Essentia — that work on being and essence written as an introduction for beginners in the schools — establishes that existence is not contained in the concept of a created essence.

Whether that conceptual non-inclusion proves a real distinction between essence and existence as two ontological components, or only a logical distinction in thought, the treatise does not settle with full explicitness.⁷ In other writings I teach the real distinction plainly. But a careful reader will notice that a parallel argument in the same work — showing that an individual and its specific nature are conceptually distinct — does not establish a real distinction between them, since Socrates and his humanity are really identical.⁸

The friends of my thought have not finished arguing this among themselves, and I do not think the argument as compressed in that early work resolves it by itself. What is not in doubt is the theological weight of the distinction. Wherever essence and existence are identical, that reality is its own existence and exists of itself; and self-existence is proper to God alone.⁹ Everything else receives its existence; God simply is.

Sources

we can form a notion of what a man or phoenix is without knowing whether or not it exists in reality. Being or existence is not contained in the definition of their essence.
On Being and Essence, pp. 7–8
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, pp. 7–8
The only possible exception to this general rule would be a being whose essence is existence itself. Clearly the notion of what this unique being is would contain existence.
On Being and Essence, pp. 7–8
of all God's names He who is (Exod. iv) is the most appropriate to him. Hence as this name is given to him in respect of his existence, it would seem that God's very existence is his essence.
On the Power of God, Quaestiones Disputatae de Potentia Dei, pp. 315–316
all created causes have one common effect which is being, although each one has its peculiar effect whereby they are differentiated: thus heat makes a thing to be hot, and a builder gives being to a house. Accordingly they have this in common that they cause being, but they differ in that fire causes fire, and a builder causes a house. There must therefore be some cause higher than all other by virtue of which they all cause being and whose proper cause is being: and this cause is God.
On the Power of God, Quaestiones Disputatae de Potentia Dei, pp. 315–316
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
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. These two intrinsic principles of a creature are said to be really distinct. But here he does not specify the kind of distinction established by this argument.
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. Consequently the similar argument for the distinction between essence and existence proves that there is a conceptual, and not necessarily a real, distinction between them.
On Being and Essence, p. 8
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