— 584 —
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, because,
      §10. The more, indeed, of these co-existing Qualities we unite
into one complex Idea, under one name, the more precise and deter-
minate we make the signification of that Word; But yet never 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 one on 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 neces-
sary 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 very 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
co-existence, 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 co-exist, can certainly be
known, unless in those of the same sense, which necessarily exclude
one another, as I have elsewhere shewed. 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
— 585 —
capable to make, or receive, on, or from other Bodies. The same
may be said of the Sound, or Taste, etc. Our specifick Names of
Substances standing for any Collections of such Ideas, ’tis not to be
wondred, that we can, with them, make very few general Proposi-
tions 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 Malleable-
ness, and the Colour or Weight of Gold, or any other part of the com-
plex 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 equal to two right ones.
Locke Hum IV, 6, §10, pp. 584-585