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.