be; because the highest probability amounts not to certainty,
without which there can be no true knowledge. For this co-existence can
be no further known than it is perceived; and it cannot be perceived but
either in particular subjects, by the observation of our senses, or, in
general, by the necessary connexion of the ideas themselves.

15. Of Repugnancy to co-exist, our knowledge is larger.

As to the incompatibility or repugnancy to co-existence, we may know
that any subject may have of each sort of primary qualities but one
particular at once: v.g. each particular extension, figure, number of
parts, motion, excludes all other of each kind. The like also is certain
of all sensible ideas peculiar to each sense; for whatever of each kind
is present in any subject, excludes all other of that sort: v.g. no one
subject can have two smells or two colours at the same time. To this,
perhaps will be said, Has not an opal, or the infusion of LIGNUM
NEPHRITICUM, two colours at the same time? To which I answer, that
these bodies, to eyes differently, placed, may at the same time afford
different colours: but I take liberty also to say, that, to eyes
differently placed, it is different parts of the object that reflect the
particles of light: and therefore it is not the same part of the object,
and so not the very same subject, which at the same time appears both
yellow and azure. For, it is as impossible that the very same particle
of any body should at the same time differently modify or reflect the
rays of light, as that it should have two different figures and textures
at the same time.

16. Our Knowledge of the Co-existence of Power in Bodies extends but a
very little Way.

But as to the powers of substances to change the sensible qualities of
other bodies, which make a great part of our inquiries about them, and
is no inconsiderable branch of our knowledge; I doubt as to these,
whether our knowledge reaches much further than our experience; or
whether we can come to the discovery of most of these powers, and be
certain that they are in any subject, by the connexion with any of those
ideas which to us make its essence. Because the active and passive
powers of bodies, and their ways of operating, consisting in a texture
and motion of parts which we cannot by any means come to discover; it is
but in very few cases we can be able to perceive their dependence on,
or repugnance to, any of those ideas which make our comple