7. For the third, we are to consider first, that upon every great agitation or concussion of the brain, as it happeneth from a stroke, especially if the stroke be upon the eye, whereby the optic nerve suffereth any great violence, there appeareth before the eyes a certain light, which light is nothing without, but an apparition only, all that is real being the concussion or motion of the parts of that nerve. From which experience we may conclude, that apparition of light without, is really nothing but motion within. If therefore from lucid bodies there can be derived motion, so as to affect the optic nerve in such manner as is proper thereunto, there will follow an image of light somewhere in that line by which the motion was last derived unto the eye; that is to say, in the object, if we look directly on it, and in the glass or water, when we look upon it in the line of reJection, which in effect is the third proposition, namely, That image and colour is but an apparition unto us of that motion, agitation, or alteration, which the object worketh in the brain, or spirits, or some internal substance in the head.
8. But that from all lucid, shining and illuminated bodies, there is a motion produced to the eye, and, through the eye, to the optic nerve, and so into the brain, by which that apparition of light or colour is effected, is not hard to prove. And first, it is evident that the fire, the only lucid body here on earth, worketh by motion equally every way; insomuch as the motion thereof stopped or inclosed, it is presently extinguished, and no more fire. And farther, that that motion, whereby the fire worketh, is dilatation, and contraction of itself alternately, commonly called scintillation or glowing, is manifest also by experience. From such motion in the fire must needs arise a rejection or casting from itself of that part of the medium which is contiguous to it, whereby that part also rejecteth the next, and so successively one part beateth back the other to the very eye; and in the same manner the exterior part of the eye (the laws of refraction still observed) presseth the interior. Now the interior coat of the eye is nothing else but a piece of the optic nerve, and therefore the motion is still continued thereby into the brain, and by resistance or reaction of the brain, is also a rebound in the optic nerve again, which we not conceiving as motion or rebound from within, think it is without, and call it light; as hath been already shewed by the experience of a stroke.
We have no reason to doubt, that the fountain of light, the sun, worketh any other wise than the fire, at least in this matter, and thus all vision hath its original from such motion as is here described. For where there is no light, there is no sight; and therefore colour also must be the same thing with light, as being the effect of lucid bodies: their difference being only this, that when the light cometh directly from the fountain to the eye, or indirectly by reflection from clean and polite bodies, and such as have no particular motion internal to alter it, we call it light. But when it cometh to the eyes by reflection from uneven, rough, and coarse bodies, or such as are affected with internal motion of their own, that may alter it, then we call it colour; colour and light differing only in this, that the one is pure, the other a perturbed light. By that which hath been said, not only the truth of the third proposition, but also the whole manner of producing light and colour, is apparent.
9. As colour is not inherent in the object, but an effect thereof upon us, caused by such motion in the object, as hath been described: so neither is sound in the thing we hear, but in ourselves. One manifest sign thereof is: that as a man may see, so also he may hear double or treble, by multiplication of echoes, which echoes are sounds as well as the original; and not being in one and the same place, cannot be inherent in the body that maketh them. Nothing can make any thing in itself: the clapper hath not sound in it, but motion, and maketh motion in the internal parts of the bell so the bell hath motion, and not sound. That imparteth motion to the air; and the air hath motion, but not sound. The air imparteth motion by the ear and nerves to the brain; and the brain hath motion but not sound. From the brain it reboundeth back into the nerves outward, and thence it becometh an apparition without, which we call sound. And to proceed to the rest of the senses, it is apparent enough, that the smell and taste of the same thing, are not the same to every man, and therefore are not in the thing smelt or tasted, but in the men. So likewise the heat we feel from the fire is manifestly in us, and is quite different from the heat that is in the fire.
For our heat is pleasure or pain, according as it is extreme or moderate; but in the coal there is no such thing. By this the fourth and last of the propositions is proved (viz.) That as in conception by vision, so also in the conceptions that arise from other senses, the subject of their inherence is not the object, but the sentient.
10. And from thence also it followeth, that whatsoever accidents or qualities our senses make us think there be in the world, they are not there, but are seemings and apparitions only.
The things that really are in the world without us, are those motions by which these seemings are caused. And this is the great deception of sense, which also is by sense to be corrected. For as sense telleth me, when I see directly, that the colour seemeth to be in the object; so also sense telleth me, when I see by reflection, that colour is not in the object.
