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 not. Secondly, the sensible
secondary 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 anything 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 alteration
a loadstone 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 use in one another, which we never suspect, because they never
appear in sensible effects.
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