things that exist, and the names they are to be ranked under?  And when general names have any connexion with particular beings, these abstract ideas are the medium that unites them:  so that the essences of species, as distinguished and denominated by us, neither are nor can be anything but those precise abstract ideas we have in our minds.  And therefore the supposed real essences of substances, if different from our abstract ideas, cannot be the essences of the species we rank things into.  For two species may be one, as rationally as two different essences be the essence of one species:  and I demand what are the alterations [which] may, or may not be made in a horse or lead, without making either of them to be of another species?  In determining the species of things by our abstract ideas, this is easy to resolve:  but if any one will regulate himself herein by supposed real essences, he will I suppose, be at a loss:  and he will never be able to know when anything precisely ceases to be of the species of a horse or lead.

14.  Each distinct abstract Idea is a distinct Essence.

Nor will any one wonder that I say these essences, or abstract ideas (which are the measures of name, and the boundaries of species) are the workmanship of the understanding, who considers that at least the complex ones are often, in several men, different collections of simple ideas; and therefore that is covetousness to one man, which is not so to another.  Nay, even in substances, where their abstract ideas seem to be taken from the things themselves, they are not constantly the same; no, not in that species which is most familiar to us, and with which we have the most intimate acquaintance:  it having been more than once doubted, whether the foetus born of a woman were a man, even so far as that it hath been debated, whether it were or were not to be nourished and baptized:  which could not be, if the abstract idea or essence to which the name man belonged were of nature’s making; and were not the uncertain and various collection of simple ideas, which the understanding put together, and then, abstracting it, affixed a name to it.  So that, in truth, every distinct abstract idea is a distinct essence; and the names that stand for such distinct ideas are the names of things essentially different.  Thus a circle is as essentially different from an oval as a sheep from a goat; and rain is as essentially different from snow as water from earth:  that abstract idea which is the essence of one being impossible to be communicated to the other.  And thus any two abstract ideas, that in any part vary one from another, with two distinct names annexed to them, constitute two distinct sorts, or, if you please, species, as essentially different as any two of the most remote or opposite in the world.

15.  Several significations of the word Essence.

But since the essences of things are thought by some (and not without reason) to be wholly unknown, it may not be amiss to consider the several significations of the word essence.