Book 4
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THE Stoics say that darkness is seen by us, for out of our eyes there issues out some light into it; and our eyes do not impose upon us, for they really perceive there is darkness. Chrysippus says that we see darkness by the striking of the intermediate air; for the visual spirits which proceed from the principal part of the soul and reach to the ball of the eye pierce this air, which, after they have made those strokes upon it, presses conically on the surrounding air, where this is homogeneous. For from the eyes those rays are poured forth which are neither black nor cloudy. Upon this account darkness is visible to us.
Chapter XVI. OF HEARING.
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EMPEDOCLES says that hearing is formed by the insidency of the air upon the spiral, which it is said hangs within the ear as a bell, and is beat upon by the air. Alcmaeon, that the vacuity that is within the ear makes us to have the sense of hearing, for the air forcing a vacuum gives the sound; every inanity affords a ringing. Diogenes, the air which is in the head, being struck upon by the voice, gives the hearing. Plato and his followers, the air which exists in the head being struck upon, is reflected to the principal part of the soul, and this causeth the sense of hearing.
Chapter XVII. OF SMELLING.
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ALCMAEON believes that the principal part of the soul, residing in the brain, draws to itself odors by respiration. Empedocles, that scents insert themselves into the breathing of the lungs; for, when there is a great difficulty in breathing, odors are not perceived by reason of the sharpness; and this we experience in those who have the defluxion of rheum.
Chapter XVIII. OF TASTE.
