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Furthermore, we find that cold can be perceived as well as heat; but mere negation cannot be seen or heard or touched or recognized by the other senses. Perception, in fact, must be of something existent; but where nothing existent is observed, privation may be inferred, being the negation of existence, as blindness is of sight, silence of sound, void and emptiness of matter. We cannot perceive a void by touch; but where no matter can be touched, void is inferred. Nor can we hear silence; yet, even though we hear nothing, we infer silence. Nor, in the same way, is sense active when things are unseen or bareAs, when a hill has been stripped of timber, you cannot see the trees.; there is, rather, inference from the negation of perception. If, therefore, cold were a privation of warmth, we ought not to be able to feel it, but only to infer it from the deficiency in warmth; but if cold is perceived by the contraction and condensation of our flesh (just as heat is by the warming and loosening of it), clearly there is some special first principle and source of coldness, just as there is of heat.
