19. Essences ingenerable and incorruptible.
That such abstract ideas, with names to them, as we have been
speaking of are essences, may further appear by what we are told concerning essences, viz., that they are all
ingenerable and incorruptible. Which cannot be true of the real constitutions of things, which begin and perish
with them. All things that exist, besides their Author, are all liable to change; especially those things we are
acquainted with, and have ranked into bands under distinct names or ensigns. Thus, that which was grass to-day is
to-morrow the flesh of a sheep; and, within a few days after, becomes part of a man: in all which and the like
changes, it is evident their real essence--i.e., that constitution whereon the properties of these several things
depended--is destroyed, and perishes with them. But essences being taken for ideas established in the mind, with
names annexed to them, they are supposed to remain steadily the same, whatever mutations the particular
substances are liable to. For, whatever becomes of Alexander and Bucephalus, the ideas to which man and horse
are annexed, are supposed nevertheless to remain the same; and so the essences of those species are preserved
whole and undestroyed, whatever changes happen to any or all of the individuals of those species. By this means
the essence of a species rests safe and entire, without the existence of so much as one individual of that kind. For,
were there now no circle existing anywhere in the world, (as perhaps that figure exists not anywhere exactly
marked out), yet the idea annexed to that name would not cease to be what it is; nor cease to be as a pattern to
determine which of the particular figures we meet with have or have not a right to the name circle, and so to show
which of them, by having that essence, was of that species. And though there neither were nor had been in nature
such a beast as an unicorn, or such a fish as a mermaid; yet, supposing those names to stand for complex abstract
ideas that contained no inconsistency in them, the essence of a mermaid is as intelligible as that of a man; and the
idea of an unicorn as certain, steady, and permanent as that of a horse. From what has been said, it is evident, that
the doctrine of the immutability of essences proves them to be only abstract ideas; and is founded on the relation
established between them and certain sounds as signs of them; and will always be true, as long as the same name
can have the same signification.