— 443 —
      §8. And that the Species of Things to us, are nothing but the ranking
them under distinct Names, according to the complex Ideas in us; and not
according to precise, distinct, real Essences in them, is plain from
hence; That we find many of the Individuals that are ranked into
one Sort, called by one common Name, and so received as being of
one Species, have yet Qualities depending on their real Constitu-
tions, as far different one from another, as from others, from which
they are accounted to differ specifically. This, as it is easy to be
observed by all, who have to do with natural Bodies; so Chymists
especially are often, by sad Experience, convinced of it, when they,
sometimes in vain, seek for the same Qualities in one parcel of
Sulphur, Antimony, or Vitriol, which they have found in others.
For though they are Bodies of the same Species, having the same
nominal Essence, under the same Name; yet do they often, upon
severe ways of examination, betray Qualities so different one from
another, as to frustrate the Expectation and Labour of very wary
Chymists. But if Things were distinguished into Species, according
to their real Essences, it would be as impossible to find different
Properties in any two individual Substances of the same Species, as
it is to find different Properties in two Circles, or two equilateral
Triangles. That is properly the Essence to us, which determines
every particular to this or that Classis; or, which is the same Thing,
— 444 —
to this or that general Name: And what can that be else, but that
abstract Idea, to which that name is annexed? and so has, in truth, a
reference, not so much to the being of particular Things, as to their
general Denominations.
Locke Hum III, 6, §8, pp. 443-444