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.

20.  Recapitulation.

To conclude.  This is that which in short I would say, viz. that all the great business of genera and species, and their essences, amounts to no more but this:—­That men making abstract ideas, and settling them in their minds with names annexed to them, do thereby enable themselves to consider things, and discourse of them, as it were in bundles, for the easier and readier improvement and communication of their knowledge, which would advance but slowly were their words and thoughts confined only to particulars.