Hyl. I mean the way of accounting for our sensations or ideas.
Phil. How is that?
Hyl. It is supposed the soul makes her residence in some part of the brain, from which the nerves take their rise, and are thence extended to all parts of the body; and that outward objects, by the different impressions they make on the organs of sense, communicate certain vibrative motions to the nerves; and these being filled with spirits propagate them to the brain or seat of the soul, which, according to the various impressions or traces thereby made in the brain, is variously affected with ideas[820].
Phil. And call you this an explication of the manner whereby we are affected with ideas?
Hyl. Why not, Philonous? Have you anything to object against it?
Phil. I would first know whether I rightly understand your hypothesis. You make certain traces in the brain to be the causes or occasions of our ideas. Pray tell me whether by the brain you mean any sensible thing.
Hyl. What else think you I could mean?
Phil. Sensible things are all immediately perceivable; and those things which are immediately perceivable are ideas; and these exist only in the mind. Thus much you have, if I mistake not, long since agreed to.
Hyl. I do not deny it.
Phil. The brain therefore you speak of, being a sensible thing, exists only in the mind[821]. Now, I would fain know whether you think it reasonable to suppose that one idea or thing existing in the mind occasions all other ideas. And, if you think so, pray how do you account for the origin of that primary idea or brain itself?