18. Men can have no ideas of real essences. Fourthly, The real
essences of those things which we distinguish into species, and as
so distinguished we name, ought to be known; i.e. we ought to have
ideas of them. But since we are ignorant in these four points, the
supposed real essences of things stand us not in stead for the
distinguishing substances into species.
19. Our nominal essences of substances not perfect collections of
the properties that flow from their real essences. Fifthly, The only
imaginable help in this case would be, that, having framed perfect
complex ideas of the properties of things flowing from their different
real essences, we should thereby distinguish them into species. But
neither can this be done. For, being ignorant of the real essence
itself, it is impossible to know all those properties that flow from
it, and are so annexed to it, that any one of them being away, we
may certainly conclude that that essence is not there, and so the
thing is not of that species. We can never know what is the precise
number of properties depending on the real essence of gold, any one of
which failing, the real essence of gold, and consequently gold,
would not be there, unless we knew the real essence of gold itself,
and by that determined that species. By the word gold here, I must
be understood to design a particular piece of matter; v.
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