Secondly, Names of simple Ideas and Modes signify always both real and nominal Essences.
Secondly, The names of simple ideas and modes signify always the real as well as nominal essence of their species.

But the names of natural substances signify rarely, if ever, anything but barely the nominal essences of those species; as we shall show in the chapter that treats of the names of substances in particular.
4.

Thirdly, Names of simple Ideas are undefinable.
Thirdly, The names of simple ideas are not capable of any definition; the names of all complex ideas are.

It has not, that I know, been yet observed by anybody what words are, and what are not, capable of being defined; the want whereof is (as I am apt to think) not seldom the occasion of great wrangling and obscurity in men's discourses, whilst some demand definitions of terms that cannot be defined; and others think they ought not to rest satisfied in an explication made by a more general word, and its restriction, (or to speak in terms of art, by a genus and difference,) when, even after such definition, made according to rule, those who hear it have often no more a clear conception of the meaning of the word than they had before.

This at least I think, that the showing what words are, and what are not, capable of definitions, and wherein consists a good definition, is not wholly besides our present purpose; and perhaps will afford so much light to the nature of these signs and our ideas, as to deserve a more particular consideration.
5.