[67a] not being derived either from many or from simple forms, but are indicated by two distinctive terms only, “pleasant” and “painful” of which the one kind roughens and violently affects the whole of our bodily cavity which lies between the head and the navel, whereas the other mollifies this same region and restores it agreeably to its natural condition.The third organ of perception within us which we have to describe in our survey is that of hearing, [67b] and the causes whereby its affections are produced. In general, then, let us lay it down that sound is a stroke transmitted through the ears, by the action of the air upon the brain and the blood, and reaching to the soul; and that the motion caused thereby, which begins in the head and ends about the seat of the liver, is “hearing”; and that every rapid motion produces a “shrill” sound, and every slower motion a more “deep” sound; and that uniform motion produces an “even” and smooth sound and the opposite kind of motion a “harsh” sound; [67c] and that large motion produces “loud” sound, and motion of the opposite kind “soft” sound. The subject of concords of sounds must necessarily be treated in a later part of our exposition.1We have still remaining a fourth kind of sensation, which we must divide up seeing that it embraces numerous varieties, which, as a whole, we call “colors.” This consists of a flame which issues from the several bodies, and possesses particles so proportioned to the visual stream as to produce sensation; and as regards the visual stream, we have already stated2 merely the causes which produced it. [67d] Concerning colors, then, the following explanation will be the most probable and worthy of a judicious account. Of the particles which fly off from the rest and strike into the visual stream some are smaller, some larger, and some equal to the particles of the stream itself; those, then, that are equal are imperceptible, and we term them “transparent”; while the larger and smaller particles—of which the one kind contracts, the other dilates the visual stream—are akin to the particles of heat and cold which affect the flesh, [67e] and to the astringent particles which affect the tongue, and to all the heating particles which we call “bitter“3 with these “white” and “black” are really identical affections, occurring in a separate class of sensation, although they appear different for the causes stated. These, therefore, are the names we must assign to them: that which dilates the visual stream is “white” and the opposite thereof “black“4; and the more rapid motion, being that of a different species of fire, which strikes upon the visual stream and dilates it as far as to the eyes, and penetrating
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