19. 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, etc. 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 versâ: 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.
But this reference of the name to a thing, whereof we have not the idea, is so far from helping at all, that it only
serves the more to involve us in difficulties. For by this tacit reference to the real essence of that species of bodies,
the word gold (which, by standing for a more or less perfect collection of simple ideas, serves to design that sort
of body well enough in civil discourse) comes to have no signification at all, being put for somewhat whereof we
have no idea at all, and so can signify nothing at all, when the body itself is away. For however it may be thought
all one, yet, if well considered, it will be found a quite different thing, to argue about gold in name, and about a
parcel in the body itself, v.g. a piece of leaf-gold laid before us; though in discourse we are fain to substitute the
name for the thing.