20. The cause of this abuse, a supposition of nature's working always regularly, in setting boundaries to species.
That which I think very much disposes men to substitute their names for the real essences of species, is the
supposition before mentioned, that nature works regularly in the production of things, and sets the boundaries to
each of those species, by giving exactly the same real internal constitution to each individual which we rank under
one general name. Whereas anyone who observes their different qualities can hardly doubt, that many of the
individuals, called by the same name, are, in their internal constitution, as different one from another as several of
those which are ranked under different specific names. This supposition, however, that the same precise and
internal constitution goes always with the same specific name, makes men forward to take those names for the
representatives of those real essences; though indeed they signify nothing but the complex ideas they have in their
minds when they use them. So that, if I may so say, signifying one thing, and being supposed for, or put in the
place of another, they cannot but, in such a kind of use, cause a great deal of uncertainty in men's discourses;
especially in those who have thoroughly imbibed the doctrine of substantial forms, whereby they firmly imagine
the several species of things to be determined and distinguished.
