15. Whilst our complex ideas of substances contain not ideas of their real constitutions, we can make but few
general certain propositions concerning them.
This is evident, the abstract complex ideas of substances. for which
their general names stand, not comprehending their real constitutions, can afford us 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
ourselves about, do depend, or with which they have any certain connexion: v.g. let the ideas to which we give the
name man be, as it commonly is, a body of the ordinary shape, with sense, voluntary motion, and reason joined 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
ourselves with probability in the rest: but can have no general certainty, whilst our specific idea of man contains
not that real constitution 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 qualities and
powers in him, there is no discernible connexion or repugnance between our specific 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 constitutions 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.
