10. As far as any such co-existence can be known, so far universal propositions may be certain. But this will go
but a little way.
The more, indeed, of these coexisting qualities we unite into one complex idea, under one name,
the more precise and determinate we make the signification of that word; but never yet make it thereby more
capable of universal certainty, in respect of other qualities not contained in our complex idea: since we perceive
not their connexion or dependence on one another; being ignorant both of that real constitution in which they are
all founded, and also how they flow from it. For the chief part of our knowledge concerning substances is not, as
in other things, barely of the relation of two ideas that may exist separately; but is of the necessary connexion and
co-existence of several distinct ideas in the same subject, or of their repugnancy so to co-exist. Could we begin at
the other end, and discover what it was wherein that colour consisted, what made a body lighter or heavier, what
texture of parts made it malleable, fusible, and fixed, and fit to be dissolved in this sort of liquor, and not in
another;--if, I say, we had such an idea as this of bodies, and could perceive wherein all sensible qualities
originally consist, and how they are produced; we might frame such abstract ideas of them as would furnish us
with matter of more general knowledge, and enable us to make universal propositions, that should carry general
truth and certainty with them. But whilst our complex ideas of the sorts of substances are so remote from that
internal real constitution on which their sensible qualities depend, and are made up of nothing but an imperfect
collection of those apparent qualities our senses can discover, there can be few general propositions concerning
substances of whose real truth we can be certainly assured; since there are but few simple ideas of whose
connexion and necessary coexistence we can have certain and undoubted knowledge. I imagine, amongst all the
secondary qualities of substances, and the powers relating to them, there cannot any two be named, whose
necessary co-existence, or repugnance to coexist, can certainly be known; unless in those of the same sense,
which necessarily exclude one another, as I have elsewhere shown. No one, I think, by the colour that is in any
body, can certainly know what smell, taste, sound, or tangible qualities it has, nor what alterations it is capable to
make or receive on or from other bodies. The same may be said of the sound or taste, etc. Our specific names of
substances standing for any collections of such ideas, it is not to be wondered that we can with them make very
few general propositions of undoubted real certainty. But yet so far as any complex idea of any sort of substances
contains in it any simple idea, whose necessary co-existence with any other may be discovered, so far universal
propositions may with certainty be made concerning it: v.g. could any one discover a necessary connexion
between malleableness and the colour or weight of gold, or any other part of the complex idea signified by that
name, he might make a certain universal proposition concerning gold in this respect; and the real truth of this
proposition, that all gold is malleable, would be as certain as of this, the three angles of all right-lined triangles are
all equal to two right ones.
