nowing WHAT
figure, size, or motion of parts produce a yellow colour, a sweet taste,
or a sharp sound, that we can by no means conceive how ANY size, figure,
or motion of any particles, can possibly produce in us the idea of any
colour, taste, or sound whatsoever: there is no conceivable connexion
between the one and the other.

14. And seek in vain for certain and universal knowledge of unperceived
qualities in substances.

In vain, therefore, shall we endeavour to discover by our ideas (the
only true way of certain and universal knowledge) what other ideas are
to be found constantly joined with that of OUR complex idea of any
substance: since we neither know the real constitution 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
inquiries reaches very little further 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 that 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 connexion of their
ideas one with another, we cannot know certainly any two to co-exist,
any further 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 cannot certainly know that where any four
of these are, the fifth will be there also, how highly probable soever
it may