18. Real and nominal essence the same in simple ideas and modes,
different in substances. Essences being thus distinguished into
nominal and real, we may further observe, that, in the species of
simple ideas and modes, they are always the same; but in substances
always quite different. Thus, a figure including a space between three
lines, is the real as well as nominal essence of a triangle; it
being not only the abstract idea to which the general name is annexed,
but the very essentia or being of the thing itself; that foundation
from which all its properties flow, and to which they are all
inseparably annexed. But it is far otherwise concerning that parcel of
matter which makes the ring on my finger; wherein these two essences
are apparently different. For, it is the real constitution of its
insensible parts, on which depend all those properties of colour,
weight, fusibility, fixedness, &c., which are to be found in it; which
constitution we know not, and so, having no particular idea of, having
no name that is the sign of it. But yet it is its colour, weight,
fusibility, fixedness, &c., which makes it to be gold, or gives it a
right to that name, which is therefore its nominal essence. Since
nothing can be called gold but what has a conformity of qualities to
that abstract complex idea to which that name is annexed.
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