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80 to such contraries as can generate from and give growth to one another. Thus, there are many things derived from one another which are not always quantities, as the healthy, for instance, is derived from the unhealthy; neither do these contraries appear, in any manner, to be nourishment for one another, as water, for instance, is nourishment for fire, but fire is not nourishment for water. It is in homogeneous bodies especially, that the contraries seem to be in the relations of nourishment and nourished. But here there is a difficulty; for while some maintain that like is nourished as it is increased by like, there are others who maintain, as we have said, that it is contrary which is nourished by contrary; that like is unimpressionable by like; that food undergoes change and is digested, and that all-change implies conversion to an opposite or an intermediate state. Nourishment, besides, is affected by the body which is nourished, although the body is not affected by the nourishment, just as the material is affected by the artisan, although he is not affected by the material; for it is the artisan alone who converts the material from a raw state into one of usefulness. There is, however, a distinction to be observed in nourishment, between its last and adventitious or its first state; if both states are nourishment, distinguished only by the one being undigested, and the other digested, then it may be correct to admit of both explanations for nutrition; for in so