29. Summary.
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 inquire further 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 further, one is
as easy 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 pry into the nature and hidden causes of those ideas.
