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Abstract Ideas are the Essences of the Genera and Species.       §12. The next thing therefore to be considered, is, What kind of
signification it is, that general Words have. For as it is evident, that they
do not signify barely one particular thing; for then they would not
be general Terms, but proper Names: so on the other side, ’tis as
evident, they do not signify a plurality; for Man and Men would
then signify the same; and the distinction of numbers (as Gram-
marians call them) would be superfluous and useless. That then
which general Words signify, is a sort of Things; and each of them
does that, by being a sign of an abstract Idea in the mind, to which
Idea, as Things existing are found to agree, so they come to be
ranked under that name; or, which is all one, be of that sort.
Whereby it is evident, that the Essences of the sorts, or (if the Latin
word pleases better) Species of Things, are nothing else but these
abstract Ideas. For the having the Essence of any Species, being that
which makes any thing to be of that Species, and the conformity to
the Idea, to which the name is annexed, being that which gives a
right to that name, the having the Essence, and the having that
Conformity, must needs be the same thing: Since to be of any
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Species, and to have a right to the name of that Species, is all one.
As for Example, to be a Man, or of the Species Man, and to have a
right to the name Man, is the same thing. Again, to be a Man, or of
the Species Man, and have the Essence of a Man, is the same thing.
Now since nothing can be a Man, or have a right to the name Man,
but what has a conformity to the abstract Idea the name Man stands
for; nor any thing be a Man, or have a right to be of the Species
Man, but what has the Essence of that Species, it follows, that the
abstract Idea, for which the name stands, and the Essence of the
Species, is one and the same. From whence it is easy to observe, that
the essences of the sorts of things, and consequently the sorting of
Things, is the Workmanship of the Understanding, since it is the
Understanding that abstracts and makes those general Ideas.
Locke Hum III, 3, §12, pp. 414-415