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Essences ingenerable and incorruptible.       §19. That such abstract Ideas, with Names to them, as we have been
speaking of, are Essences, may farther appear by what we are told
concerning Essences, viz. that they are all ingenerable, and incorrup-
tible. 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 few days after, becomes
part of a Man: In all which, and the like Changes, ’tis evident, their
real Essence, i.e. that Constitution, whereon the Properties of these
several things depended, is destroy’d, 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 undestroy’d, 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 any where in the
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World, (as, perhaps, that Figure exists not any where 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 shew 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, nor 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.
Locke Hum III, 3, §19, pp. 419-420