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.
20.