— 442 —
      §6. ’Tis true, I have often mentioned a real Essence, distinct in
Substances, from those abstract Ideas of them, which I call their
nominal Essence. By this real Essence, I mean, that real constitution of
any Thing, which is the foundation of all those Properties, that are
combined in, and are constantly found to co-exist with the nominal
Essence; that particular constitution, which every Thing has within
it self, without any relation to any thing without it. But Essence,
even in this sense, relates to a Sort, and supposes a Species: For being
that real Constitution, on which the Properties depend, it neces-
sarily supposes a sort of Things, Properties belonging only to
Species, and not to Individuals; v.g. Supposing the nominal Essence
of Gold, to be Body of such a peculiar Colour and Weight, with
Malleability and Fusibility, the real Essence is that Constitution of
the parts of Matter, on which these Qualities, and their Union,
depend; and is also the foundation of its Solubility in Aqua Regia,
and other Properties accompanying that complex Idea. Here are
Essences and Properties, but all upon supposition of a Sort, or general
abstract Idea, which is considered as immutable: but there is no
individual parcel of Matter, to which any of these Qualities are so
annexed, as to be essential to it, or inseparable from it. That which is
essential, belongs to it as a Condition, whereby it is of this or that
Sort: But take away the consideration of its being ranked under the
name of some abstract Idea, and then there is nothing necessary to
it, nothing inseparable from it. Indeed, as to the real Essences of
Substances, we only suppose their Being, without precisely know-
ing what they are: But that which annexes them still to the Species,
is the nominal Essence, of which they are the supposed foundation
and cause.
Locke Hum III, 6, §6, p. 442