— 581 —
This more particularly concerns Substances.
      §5. The Names of Substances then, whenever made to stand for Species,
which are supposed to be constituted by real Essences, which we know not,
are not capable to convey Certainty to the Understanding: Of the Truth of
general Propositions made up of such Terms we cannot be sure.
The reason whereof is plain. For how can we be sure that this or
that quality is in Gold, when we know not what is or is not Gold.
Since in this way of speaking nothing is Gold, but what partakes of
an Essence, which we not knowing, cannot know where it is, or is
not, and so cannot be sure, that any parcel of Matter in the World
is or is not in this sense Gold; being incurably ignorant, whether it
has or has not that which makes any thing to be called Gold, i.e.
that real Essence of Gold whereof we have no Idea at all. This being
as impossible for us to know, as it is for a blind Man to tell in what
Flower the Colour of a pansie is, or is not to be found, whilst he has
no Idea of the Colour of a pansie at all. Or if we could (which is
impossible) certainly know where a real Essence, which we know
not, is, v.g. in what parcels of matter the real Essence of Gold is, yet
could we not be sure, that this or that quality could with truth be
affirm’d of Gold; since it is impossible for us to know, that this or
— 582 —
that quality or Idea has a necessary connexion with a real Essence,
of which we have no Idea at all, whatever Species that supposed real
Essence may be imagined to constitute.
Locke Hum IV, 6, §5, pp. 581-582