— 580 —
No Proposition can be known to be true, where the Essence of each Species mentioned is not known.
      §4. Now because we cannot be certain of the Truth of any general
Proposition, unless we know the precise bounds and extent of the Species its
Terms stand for, it is necessary we should know the Essence of each
Species, which is that which constitutes and bounds it. This, in all
simple Ideas and Modes, is not hard to do. For in these, the real and
nominal Essence being the same; or which is all one, the abstract
Idea, which the general Term stands for, being the sole Essence and
Boundary, that is or can be supposed, of the Species, there can be no
doubt, how far the Species extends, or what Things are compre-
hended under each Term: which, ’tis evident, are all, that have an
exact conformity with the Idea it stands for, and no other. But in
Substances, wherein a real Essence, distinct from the nominal, is
supposed to constitute, determine, and bound the Species, the
extent of the general Word is very uncertain: because not knowing
this real Essence, we cannot know what is, or is not of that Species;
and consequently what may, or may not with certainty be affirmed
of it. And thus speaking of a Man, or Gold, or any other Species
of natural Substances, as supposed constituted by a precise real
Essence, which Nature regularly imparts to every individual of that
Kind, whereby it is made to be of that Species, we cannot be certain
of the truth of any Affirmation of Negation made of it. For Man,
or Gold, taken in this sense, and used for Species of Things, consti-
tuted by real Essences, different from the complex Idea in the Mind of
the Speaker, stand for we know not what: and the extent of these
Species, with such Boundaries, are so unknown and undetermined,
that it is impossible, with any certainty, to affirm, that all Men are
rational, or that all Gold is yellow. But where the nominal Essence
is kept to, as the Boundary of each Species, and Men extend the
application of any general Term no farther than to the particular
Things, in which the complex Idea it stands for is to be found,
there they are in no danger to mistake the bounds of each Species,
nor can be in doubt, on this account, whether any Proposition be
true, or no. I have chose to explain this uncertainty of Propositions
— 581 —
in this scholastick way, and have made use of the Terms of Essences
and Species, on purpose to shew the absurdity and inconvenience
there is to think of them, as of any other sort of Realities, than
barely abstract Ideas with Names to them. To suppose, that the
Species of Things are any thing, but the sorting of them under
general Names, according as they agree to several abstract Ideas, of
which we make those Names the Signs, is to confound Truth,
and introduce Uncertainty into all general Propositions, that can
be made about them. Though therefore these Things might, to
People not possessed with scholastick Learning, be perhaps treated
of, in a better and clearer way: yet those wrong Notions of Essences
and Species, having got root in most Peoples Minds, who have
received any tincture from the Learning, which has prevailed
in this part of the World, are to be discovered and removed, to
make way for that use of Words, which should convey certainty
with it.
Locke Hum IV, 6, §4, pp. 580-581