Indeed, as to the real
essences of substances, we only suppose their being, without precisely
knowing what they are; but that which annexes them still to the species
is the nominal essence, of which they are the supposed foundation and
cause.

7. The nominal Essence bounds the Species to us.

The next thing to be considered is, by which of those essences it is
that substances are determined into sorts or species; and that, it is
evident, is by the nominal essence. For it is that alone that the name,
which is the mark of the sort, signifies. It is impossible, therefore,
that anything should determine the sorts of things, which WE rank under
general names, but that idea which that name is designed as a mark for;
which is that, as has been shown, which we call nominal essence. Why
do we say this is a horse, and that a mule; this is an animal, that an
herb? How comes any particular thing to be of this or that sort, but
because it has that nominal essence; or, which is all one, agrees to
that abstract idea, that name is annexed to? And I desire any one but
to reflect on his own thoughts, when he hears or speaks any of those or
other names of substances, to know what sort of essences they stand for.

8. The nature of Species as formed by us.

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 constitutions,
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 chemists 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 chemists. But if things were distinguished into species,
according to their real essences, it would be as impossible to find
different pro