things that exist are cast, and do equally partake, has, I imagine, very much perplexed the knowledge of natural things.  The frequent productions of monsters, in all the species of animals, and of changelings, and other strange issues of human birth, carry with them difficulties, not possible to consist with this hypothesis; since it is as impossible that two things partaking exactly of the same real essence should have different properties, as that two figures partaking of the same real essence of a circle should have different properties.  But were there no other reason against it, yet the supposition of essences that cannot be known; and the making of them, nevertheless, to be that which distinguishes the species of things, is so wholly useless and unserviceable to any part of our knowledge, that that alone were sufficient to make us lay it by, and content ourselves with such essences of the sorts or species of things as come within the reach of our knowledge:  which, when seriously considered, will be found, as I have said, to be nothing else but, those abstract complex ideas to which we have annexed distinct general names.

18.  Real and nominal Essence

Essences being thus distinguished into nominal and real, we may further observe, that, in the species of simple ideas and modes, they are always the same; but in substances always quite different.  Thus, a figure including a space between three lines, is the real as well as nominal essence of a triangle; it being not only the abstract idea to which the general name is annexed, but the very ESSENTIA or being of the thing itself; that foundation from which all its properties flow, and to which they are all inseparably annexed.  But it is far otherwise concerning that parcel of matter which makes the ring on my finger; wherein these two essences are apparently different.  For, it is the real constitution of its insensible parts, on which depend all those properties of colour, weight, fusibility, fixedness, &c., which are to be found in it; which constitution we know not, and so, having no particular idea of, having no name that is the sign of it.  But yet it is its colour, weight, fusibility, fixedness, &c., which makes it to be gold, or gives it a right to that name, which is therefore its nominal essence.  Since nothing can be called gold but what has a conformity of qualities to that abstract complex idea to which that name is annexed.  But this distinction of essences, belonging particularly to substances, we shall, when we come to consider their names, have an occasion to treat of more fully.

19.  Essences ingenerable and incorruptible.