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, &c. 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 existence with any
other may be discovered, so far universal propositions may with
certainty be made concerning it: v.
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