2. The essence of each sort of substance is our abstract idea to
which the name is annexed. 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
everything 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 distinguish it from the 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.
3. The nominal and real essence different. For, though perhaps
voluntary motion, with sense and reason, joined to a body of a certain
shape, be the complex idea to which I and others annex the name man,
and so be the nominal essence of the species so called: yet nobody
will say that complex idea is the real essence and source of all those
operations which are to be found in any individual of that sort.
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