Hence we think Change of our Complex Ideas of Substances not to change their Species.
This shows us the reason why in MIXED MODES any of the ideas that make the composition of the complex one being left out or changed, it is allowed to be another thing, i.e.to be of another species, as is plain in CHANCE-MEDLEY, MANSLAUGHTER, MURDER, PARRICIDE, &c.

The reason whereof is, because the complex idea signified by that name is the real as well as nominal essence; and there is no secret reference of that name to any other essence but that.

But in SUBSTANCES, it is not so.

For though in that called GOLD, one puts into his complex idea what another leaves out, and vice versa: yet men do not usually think that therefore the species is changed: because they secretly in their minds refer that name, and suppose it annexed to a real immutable essence of a thing existing, on which those properties depend.

He that adds to his complex idea of gold that of fixedness and solubility in AQUA REGIA, which he put not in it before, is not thought to have changed the species; but only to have a more perfect idea, by adding another simple idea, which is always in fact joined with those other, of which his former complex idea consisted.