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.

30.  Our idea of Spirit and our idea of Body compared.

So that, in short, the idea we have of spirit, compared with the idea we have of body, stands thus:  the substance of spirits is unknown to us; and so is the substance of body equally unknown to us.  Two primary qualities or properties of body, viz. solid coherent parts and impulse, we have distinct clear ideas of:  so likewise we know, and have distinct clear ideas, of two primary qualities or properties of spirit, viz. thinking, and a power of action; i.e. a power of beginning or stopping several thoughts or motions.  We have also the ideas of several qualities inherent in bodies, and have the clear distinct ideas of them; which qualities are but the various modifications of the extension of cohering solid parts, and their motion.  We have likewise the ideas of the several modes of thinking viz. believing, doubting, intending, fearing, hoping; all which are but the several modes of thinking.  We have also the ideas of willing, and moving the body consequent to it, and with the body itself too; for, as has been shown, spirit is capable of motion.

31.  The Notion of Spirit involves no more Difficulty in it than that of Body.