— 439 —
The Essence of each sort is the abstract Idea.       §2. The measure and boundary of each Sort, or Species, whereby
it is constituted that particular Sort, and distinguished from others,
is that we call its Essence, which is nothing but that abstract Idea to
which the Name is annexed: So that every thing contained in that Idea,
is essential to that Sort. This, though it be all the Essence of natural
Substances, that we know, or by which we distinguish them into
Sorts; yet I call it by a peculiar name, the nominal Essence, to dis-
tinguish it from that real Constitution of Substances, upon which
depends this nominal Essence, and all the Properties of that Sort;
which therefore, as has been said, may be called the real Essence: v.g.
the nominal Essence of Gold, is that complex Idea the word Gold stands
for, let it be, for instance, a Body yellow, of a certain weight,
malleable, fusible, and fixed. But the real Essence is the constitution
of the insensible parts of that Body, on which those Qualities, and
all the other Properties of Gold depend. How far these two are
different, though they are both called Essence, is obvious, at first
sight, to discover.
Locke Hum III, 6, §2, p. 439