— 545 —
      §14. In vain therefore shall we endeavour to discover by our
Ideas, (the only true way of certain and universal Knowledge,) what
— 546 —
other Ideas are to be found constantly joined with that of our com-
plex Idea of any Substance: since we neither know the real Con-
stitution of the minute Parts, on which their Qualities do depend;
nor, did we know them, could we discover any necessary connexion
between them, and any of the secondary Qualities: which is necessary
to be done, before we can certainly know their necessary co-existence.
So that let our complex Idea of any Species of Substances, be what it
will, we can hardly, from the simple Ideas contained in it, certainly
determine the necessary co-existence of any other Quality whatsoever.
Our Knowledge in all these Enquiries, reaches very little farther
than our Experience. Indeed, some few of the primary Qualities
have a necessary dependence, and visible connexion one with
another, as Figure necessarily supposes Extension, receiving or
communicating Motion by impulse, supposes Solidity. But though
these, and perhaps some others of our Ideas have: yet there are so
few of them, that have a visible Connexion one with another, that we
can by Intuition or Demonstration, discover the co-existence of
very few of the Qualities are to be found united in Substances: and
we are left only to the assistance of our Senses, to make known to us,
what Qualities they contain. For of all the Qualities that are co-
existent in any Subject, without this dependence and evident con-
nexion of their Ideas one with another, we cannot know certainly
any two to co-exist any farther, than Experience, by our Senses,
informs us. Thus though we see the yellow Colour, and upon trial
find the Weight, Malleableness, Fusibility, and Fixedness, that are
united in a piece of Gold; yet because no one of these Ideas has any
evident dependence, or necessary connexion with the other, we can-
not certainly know, that where any four of these are, the fifth will
be there also, how highly probable soever it may be: Because the
highest Probability, amounts not to Certainty; without which,
there can be no true Knowledge. For this co-existence can be no
farther known, than it is perceived; and it cannot be perceived but
either in particular Subjects, by the observation of our Senses, or in
general, by the necessary connexion of the Ideas themselves.
Locke Hum IV, 3, §14, pp. 545-546