— 589 —
Whilst our Ideas of Substances contain not their real Constitutions, we can make but few general Propositions concerning them.
      §15. This is evident, the abstract complex Ideas of Substances, for
which their general Names stand, not comprehending their real
Constitutions, can afford us but very little universal Certainty. Because
our Ideas of them are not made up of that, on which those Qualities
we observe in them, and would inform our selves about, do depend,
or with which they have any certain connexion. V.g. Let the Idea to
which we give the name Man, be, as it commonly is, a Body of the
ordinary shape, with Sense, voluntary Motion, and Reason join’d
— 590 —
to it. This being the abstract Idea, and consequently the Essence
of our Species Man, we can make but very few general certain
Propositions concerning Man, standing for such an Idea. Because
not knowing the real Constitution on which Sensation, power of
Motion, and Reasoning, with that peculiar Shape, depend, and
whereby they are united together in the same Subject, there are
very few other Qualities, with which we can perceive them to have
a necessary connexion: and therefore we cannot with Certainty
affirm, That all Men sleep by intervals; That no Man can be nourished by
Wood or Stones; That all Men will be poisoned by Hemlock: because
these Ideas have no connexion nor repugnancy with this our
nominal Essence of Man, with this abstract Idea that Name stands
for. We must in these and the like appeal to trial in particular
Subjects, which can reach but a little way. We must content our
selves with Probability in the rest: but can have no general Cer-
tainty, whilst our specifick Idea of Man, contains not that real Con-
stitution, which is the root, wherein all his inseparable Qualities are
united, and from whence they flow. Whilst our Idea, the word Man
stands for, is only an imperfect Collection of some sensible Quali-
ties and Powers in him, there is no discernible connexion or repug-
nance between our specifick Idea, and the Operation of either the
Parts of Hemlock or Stones, upon his Constitution. There are
Animals that safely eat Hemlock, and others that are nourished by
Wood and Stones: But as long as we want Ideas of those real Consti-
tutions of different sorts of Animals, whereon these, and the like
Qualities and Powers depend, we must not hope to reach Certainty
in universal Propositions concerning them. Those few Ideas only,
which have a discernible connexion with our nominal Essence, or
any part of it, can afford us such Propositions. But these are so few,
and of so little moment, that we may justly look on our certain
general Knowledge of Substances, as almost none at all.
Locke Hum IV, 6, §15, pp. 589-590