To distinguish substantial beings into species, according to the usual supposition, that there are certain precise essences or forms of things, whereby all the individuals existing are, by nature distinguished into species, these things are necessary:—­

15.  A crude supposition.

First, To be assured that nature, in the production of things, always designs them to partake of certain regulated established essences, which are to be the models of all things to be produced.  This, in that crude sense it is usually proposed, would need some better explication, before it can fully be assented to.

16.  Monstrous births.

Secondly, It would be necessary to know whether nature always attains that essence it designs in the production of things.  The irregular and monstrous births, that in divers sorts of animals have been observed, will always give us reason to doubt of one or both of these.

17.  Are monsters really a distinct species?

Thirdly, It ought to be determined whether those we call monsters be really a distinct species, according to the scholastic notion of the word species; since it is certain that everything that exists has its particular constitution.  And yet we find that some of these monstrous productions have few or none of those qualities which are supposed to result from, and accompany, the essence of that species from whence they derive their originals, and to which, by their descent, they seem to belong.

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 the Real Essence.

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. g. the last guinea that was coined.  For, if it should stand here, in its ordinary signification, for that complex idea which I or any one else calls gold, i. e. for the nominal essence of gold, it would be jargon.  So hard is it to show the various meaning and imperfection of words, when we have nothing else but words to do it by.