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Three sorts of Ideas make our complex ones of Substances.       §9. The Ideas that make our complex ones of corporeal Substances, are of
these three sorts. First, The Ideas of the primary Qualities of things,
which are discovered by our Senses, and are in them even when we
perceive them not, such are the Bulk, Figure, Number, Situation,
and Motion of the parts of Bodies, which are really in them,
whether we take notice of them or no. Secondly, The sensible secon-
dary Qualities, which depending on these, are nothing but the
Powers, those Substances have to produce several Ideas in us by our
Senses; which Ideas are not in the things themselves, otherwise than
as any thing is in its Cause. Thirdly, The aptness we consider in any
Substance, to give or receive such alterations of primary Qualities,
as that the Substance so altered, should produce in us different
Ideas from what it did before, these are called active and passive
Powers: All which Powers, as far as we have any Notice or Notion
of them, terminate only in sensible simple Ideas. For whatever
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alteration a Load-stone has the Power to make in the minute Particles
of Iron, we should have no Notion of any Power it had at all to
operate on Iron, did not its sensible Motion discover it; and I doubt
not, but there are a thousand Changes, that Bodies we daily handle,
have a Power to cause in one another, which we never suspect,
because they never appear in sensible effects.
Locke Hum II, 23, §9, pp. 300-301