Another idea we have of body is, the power of communication of motion by impulse; and of our souls, the power of exciting motion by thought.  These ideas, the one of body, the other of our minds, every day’s experience clearly furnishes us with:  but if here again we inquire how this is done, we are equally in the dark.  For, in the communication of motion by impulse, wherein as much motion is lost to one body as is got to the other, which is the ordinariest case, we can have no other conception, but of the passing of motion out of one body into another; which, I think, is as obscure and inconceivable as how our minds move or stop our bodies by thought, which we every moment find they do.  The increase of motion by impulse, which is observed or believed sometimes to happen, is yet harder to be understood.  We have by daily experience clear evidence of motion produced both by impulse and by thought; but the manner how, hardly comes within our comprehension:  we are equally at a loss in both.  So that, however we consider motion, and its communication, either from body or spirit, the idea which belongs to spirit is at least as clear as that which belongs to body.  And if we consider the active power of moving, or, as I may call it, motivity, it is much clearer in spirit than body; since two bodies, placed by one another at rest, will never afford us the idea of a power in the one to move the other, but by a borrowed motion:  whereas the mind every day affords us ideas of an active power of moving of bodies; and therefore it is worth our consideration, whether active power be not the proper attribute of spirits, and passive power of matter.  Hence may be conjectured that created spirits are not totally separate from matter, because they are both active and passive.  Pure spirit, viz.  God, is only active; pure matter is only passive; those beings that are both active and passive, we may judge to partake of both.  But be that as it will, I think, we have as many and as clear ideas belonging to spirit as we have belonging to body, the substance of each being equally unknown to us; and the idea of thinking in spirit, as clear as of extension in body; and the communication of motion by thought, which we attribute to spirit, is as evident as that by impulse, which we ascribe to body.  Constant experience makes us sensible of both these, though our narrow understandings can comprehend neither.  For, when the mind would look beyond those original ideas we have from sensation or reflection, and penetrate into their causes, and manner of production, we find still it discovers nothing but its own short-sightedness.

29.  Summary.