Metaphysics — Essences
The essence of a thing is what that thing is necessarily and primarily as the first principle of its intelligibility. (ItP)
Previously, we saw that the object of the intellect was being, meaning that essence refers to being of something insofar as it is intelligible. The essence of man is “the rational animal”. Note that strictly speaking, only universals are intelligible — this fact becomes more apparent when we study Arisotelian-Thomistic psychology. Suffice it to say, that, when you think of a particular man, say Socrates, what is first intelligible is that he possesses the essence of man, and later that he possesses the accidents of being Greek, being a philosopher, etc. The primary thing which Socrates is is man. While characteristics like being Greek and being a philosopher are inseparable from the individuality of Socrates, they are not primary characteristics, and they can be removed without affecting his being as man.
As Maritain says:
We must therefore conclude that these characteristics have their ground in what the object is necessarily and primarily, but as an individual, or, in what we may term the individual nature of the thing. (By individual nature we mean incommunicable to any other object, or if you prefer, wholly circumscribed.) (ItP)
Maritain says that essences are neither individual nor universal. They are individual, since they really exist in every individual we perceive them in. They are universal, since they exist universally in the mind. The way that they are so is a more complex topic for a later time.
This is a brief summary of essence, also known as quiddity, or nature.