9. Three sorts of ideas make our complex ones of corporeal substances.
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.
