— 312 —
      §29. To conclude, Sensation convinces us, that there are solid
extended Substances; and Reflection, that there are thinking ones:
Experience assures us of the Existence of such Beings; and that the
one hath a power to move Body by impulse, the other by thought;
this we cannot doubt of. Experience, I say, every moment furnishes
us with the clear Ideas, both of the one, and the other. But beyond
these Ideas, as received from their proper Sources, our Faculties will
not reach. If we would enquire farther into their Nature, Causes,
and Manner, we perceive not the Nature of Extension, clearer than
we do of Thinking. If we would explain them any farther, one is as
easie as the other; and there is no more difficulty, to conceive how
a Substance we know not, should by thought set Body into motion,
than how a Substance we know not, should by impulse set Body into
motion. So that we are no more able to discover, wherein the Ideas
belonging to Body consist, than those belonging to Spirit. From
whence it seems probable to me, that the simple Ideas we receive
from Sensation and Reflection, are the Boundaries of our Thoughts;
beyond which, the Mind, whatever efforts it would make, is not
able to advance one jot; nor can it make any discoveries, when it
would prie into the Nature and hidden Causes of those Ideas.
Locke Hum II, 23, §29, p. 312