of its figure.

Hence arises the certainty of the mathematical sciences, as they explain
these properties of bodies, which are exactly resembled by our ideas of
them, whilst we are obliged to collect almost all our other knowledge from
experiment; that is, by observing the effects exerted by one body upon
another.

3. _Of the Penetrability of Matter._

The impossibility of two bodies existing together in the same space cannot
be deduced from our idea of solidity, or of figure. As soon as we perceive
the motions of objects that surround us, and learn that we possess a power
to move our own bodies, we experience, that those objects, which excite in
us the idea of solidity and of figure, oppose this voluntary movement of
our own organs; as whilst I endeavour to compress between my hands an ivory
ball into a spheroid. And we are hence taught by experience, that our own
body and those, which we touch, cannot exist in the same part of space.

But this by no means demonstrates, that no two bodies can exist together in
the same part of space. Galilæo in the preface to his works seems to be of
opinion, that matter is not impenetrable; Mr. Michel, and Mr. Boscowich in
his Theoria. Philos. Natur. have espoused this hypothesis: which has been
lately published by Dr. Priestley, to whom the world is much indebted for
so many important discoveries in science. (Hist. of Light and Colours, p.
391.) The uninterrupted passage of light through transparent bodies, of the
electric æther through metallic and aqueous bodies, and of the magnetic
effluvia through all bodies, would seem to give some probability to this
opinion. Hence it appears, that beings may exist without possessing the
property of solidity, as well as they can exist without possessing the