4. No proposition can be certainly known to be true, where the real essence of each species mentioned is not
known.
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
comprehended under each term; which, it is 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 what 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 and 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 or negation made of it. For
man or gold, taken in this sense, and used for species of things constituted 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 further 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 not. I have chosen to explain this uncertainty of propositions
in this scholastic way, and have made use of the terms of essences, and species, on purpose to show 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 anything 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 scholastic learning, be treated of in a better and clearer way; yet those wrong
notions of essences or species having got root in most people's 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.