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assimilate what is proper for them, and reject what is hurt 

 ful ; or when a sponge attracts water, and expels air, etc. ; 

 for in all cases, no one body placed near to another can 

 change that other, or be changed by it, unless a reciprocal 

 perception precede the operation. A body always perceives 

 the passages by which it insinuates; feels the impulse of 

 another body, where it yields thereto; perceives the re 

 moval of any body that withheld it, and thereupon 

 recovers itself; perceives the separation of its continuity, 

 and for a time resists it; in fine, perception is diffused 

 through all nature. But air has such an acute perception 

 of heat and cold, as far exceeds the human touch, which 

 yet passes for the measure of heat and cold. This doctrine, 

 therefore, has two defects: one, in that men have generally 

 passed it over untouched, though a noble subject; the other, 

 that they who did attend to it have gone too far, attributed 

 sense to all bodies, and made it almost a sin to pluck a twig 

 from a tree, lest the tree should groan, like Polydorus in 

 Virgil. 8 But they ought carefully to have searched after 

 the difference between perception and sense; not only in 

 comparing sensible with insensible things, in the entire 

 bodies thereof, as those of plants and animals, but also to 

 have observed in the sensible body itself, what should be 

 the cause that so many actions are performed without any 

 sense at all. Why the aliments are digested and discharged, 

 the humors and juices carried up and down in the body; why 

 the heart and pulse beat; why the viscera act as so many 

 workshops, and each performs its respective office; yet all 

 this, and much more, be done without sense. But men 

 have not yet sufficiently found of what nature the action 

 of sense is, and what kind of body, what continuance, what 

 repetitions of the impression are required to cause pain or 

 pleasure. Lastly, they seem totally ignorant of the differ 

 ence between simple perception and sense, and how far 

 perception may be caused without sense. Nor is this a 



8 Virg. ^Eneid, iii. 



