But if that particular being be to be counted of the sort MAN, and to have the name MAN given it, then reason is essential to it; supposing reason to be a part of the complex idea the name man stands for: as it is essential to this thing I write on to contain words, if I will give it the name TREATISE, and rank it under that species.

So that essential and not essential relate only to our abstract ideas, and the names annexed to them; which amounts to no more than this, That whatever particular thing has not in it those qualities which are contained in the abstract idea which any general term stands for, cannot be ranked under that species, nor be called by that name; since that abstract idea is the very essence of that species.
5.

The only essences perceived by us in individual substances are those qualities which entitle them to receive their names.
Thus, if the idea of BODY with some people be bare extension or space, then solidity is not essential to body: if others make the idea to which they give the name BODY to be solidity and extension, then solidity is essential to body.

That therefore, and that alone, is considered as essential, which makes a part of the complex idea the name of a sort stands for; without which no particular thing can be reckoned of that sort, nor be entitled to that name.

Should there be found a parcel of matter that had all the other qualities that are in iron, but wanted obedience to the loadstone, and would neither be drawn by it nor receive direction from it, would any one question whether it wanted anything essential?
It would be absurd to ask, Whether a thing really existing wanted anything essential to it.