12. Because necessary connexion between any secondary and the
primary qualities is undiscoverable by us. Besides this ignorance of
the primary qualities of the insensible parts of bodies, on which
depend all their secondary qualities, there is yet another and more
incurable part of ignorance, which sets us more remote from a
certain knowledge of the co-existence or inco-existence (if I may so
say) of different ideas in the same subject; and that is, that there
is no discoverable connexion between any secondary quality and those
primary qualities which it depends on.
13. We have no perfect knowledge of their primary qualities. That
the size, figure, and motion of one body should cause a change in
the size, figure, and motion of another body, is not beyond our
conception; the separation of the parts of one body upon the intrusion
of another; and the change from rest to motion upon impulse; these and
the like seem to have some connexion one with another. And if we
knew these primary qualities of bodies, we might have reason to hope
we might be able to know a great deal more of these operations of them
one upon another: but our minds not being able to discover any
connexion betwixt these primary qualities of bodies and the sensations
that are produced in us by them, we can never be able to establish
certain and undoubted rules of the consequence or co-existence of
any secondary qualities, though we could discover the size, figure, or
motion of those invisible parts which immediately produce them.
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