|Merlin and Glenn,|
Aristotle wrote that "a definition is a phrase signifying a thing's essence" (Top. 101b37). Rand concurred. For Aristotle the essence of a thing might be made plain to the senses or it might be established as a hypothesis (Metaph. 1025b11). Rand did not dispute that.
Fundamentally, "the essence of each thing is what it is said to be in virtue of itself. For being you is not being musical; for you are not musical in virtue of yourself. What, then, you are in virtue of yourself is your essence" (1029b14-16). For Aristotle the essential predicates of a thing say what it is, what it is to be it. To say that man is musical does not say what man is. It says something truly of man, but it does not say what is man. Moreover, for Aristotle the essence of a thing is its characteristic on which its other characteristics depend (in some sense).
Thus far, Rand concurs: "A definition must identify the nature of the units [subsumed under the concept being defined], i.e., the essential characteristics without which the units would not be the kind of existents they are" (IOE 42). Moreover, the essential characteristic of a kind under a concept is "the fundamental characteristic without which the others would not be possible. . . . Metaphysically, a fundamental characteristic is that distinctive characteristic which makes the greatest number of others possible; epistemologically, it is the one that explains the greatest number of others" (IOE 45).
Aristotle held that all natural bodies are a composite of matter and form. [Rand rejects this component of his metaphysics (IOEapdx. 286).] Typically, he took form, rather than matter, to be what makes a thing the kind of thing it is. Essence is a form.
"Aristotle held that definitions refer to metaphysical essences, which exist in concretes as a special element or formative power. . . . Aristotle regarded 'essence' as metaphysical; Objectivism regards it as epistemological" (IOE 52). For Aristotle what makes gold gold or an animal cell an animal cell is a metaphysical essence, a metaphysical form. This, Glenn, is the sort of metaphysical essence I was contrasting to physical essence.
The essence of the chemical element gold, that in virtue of which it is gold, is: having such-and-such numbers of protons and neutrons bound in a nucleus and the electrons about it. That is what makes its further distinctive properties possible. The essence of a living animal cell is that it offsets the potentially catastrophic drive of water inward through its wall by pumping sodium ions out through its wall. That is what makes possible its further distinctive properties (distinctive, say, from a living plant cell). These essences are physical. The essence of a human being---rational animality---is physical and mental. These are all essences in Rand's sense. They are physical or mental, but not metaphysical in the form-sense of Aristotle's essences.
For Rand "an essential characteristic is factual, in the sense that it does exist, does determine other characteristics, and does distinguish a group of existents from all others; it is epistemological in the sense that the classification of 'essential characteristic' is a device of man's method of cognition" (IOE 52).
Proper definition, in Rand's view, is always based on real essential characteristics. She is not so restrictive as Aristotle as to what counts as an essential characteristic. So unlike Aristotle and Locke, she does not conclude that we sometimes need to settle for a "nominal" essence, rather than a real one. Locke's nominal essence is not what Rand means in saying that essence is epistemological (rather than Aristotle-style metaphysical).