>>17813078Essence is what something is (to ti en einai). But there are differences in how essence has been conceived by philosophers throughout the centuries. For Plato, essence was identical to form, which he saw as the substance of particular things. But for Aristotle, there are actually two different types of substances: primary substances and secondary substances. Secondary substances are defined by Aristotle as the genre of primary substance. So Socrates is a concrete existing primary substance who predicates secondary substance e.g. "Socrates is a man".
But Aristotle thinks that essence is immanent in primary substance, and this is the form of them. Unlike Plato he doesn't think essence exists apart from the particulars that instantiate them, partly because he thinks it's contradictory for universal substance to be able to partake in mutually exclusive properties at once e.g. Socrates is standing but Plato is sitting. Thus substance, and by consequence essence, cannot be universal in the sense that Plato thinks, but must be immanent in concrete, individual, existing things.
Typically, Aristotelians and their derivatives (Thomists and the like) distinguish between essence and existence. Existence is that which actualizes being, and for being to be actualized, it have substance prior to it logically, or else what are you even actualizing? Modern philosophers still tend to agree with Aristotle about this, that essence precedes existence and that existence is a special property of beings that can't be predicated of them in the way other qualities are. The way Aristotle defines essence and substance is all a bit confusing though and there's still debate over what exactly he means, how and if there really is a difference in his thought, or if he's even consistent with the way he uses the terms.