Since, therefore, great difference of nature obtains in all animals, and the very nourishment which is accordant with nature is varied to suit each kind of animal, and the body which is nourished; and as in the nourishment of every animal there is a threefold cleansing and separation, it follows that whatever is alien from the nourishment of the animal must be wholly destroyed and carried off to its natural place, or change into something else, since it cannot coalesce with it; that the power of the nourishing body must be suitable to the nature of the animal to be nourished, and accordant with its powers; and that this, when it has passed through the strainers appointed for the purpose, and been thoroughly purified by the natural means of purification, must become a most genuine addition to the substance,—the only thing, in fact, which any one calling things by their right names would call nourishment at all; because it rejects everything that is foreign and hurtful to the constitution of the animal nourished and that mass of superfluous food introduced merely for filling the stomach and gratifying the appetite. This nourishment, no one can doubt, becomes incorporated with the body that is nourished, interwoven and blended with all the members and parts of members; but that which is different and contrary to nature is speedily corrupted if brought into contact with a stronger power, but easily destroys that which is overcome by it, and is converted into hurtful humours and poisonous qualities, because producing nothing akin or friendly to the body which is to be nourished. And it is a very clear proof of this, that in many of the animals nourished, pain, or disease, or death follows from these things, if, owing to a too keen appetite, they take in mingled with their food something poisonous and contrary to nature; which, of course, would tend to the utter destruction of the body to be nourished, since that which is nourished is nourished by substances akin to it and which accord with its nature, but is destroyed by those of a contrary kind. If, therefore, according to the different nature of animals, different kinds of food have been provided suitable to their nature, and none of that which the animal may have taken, not even an accidental part of it, admits of being blended with the body which is nourished, but only that part which has been purified by an entire digestion, and undergone a complete change for union with a particular body, and adapted to the parts which are to receive nourishment,—it is very plain that none of the things contrary to nature can be united with those bodies for which it is not a suitable and correspondent nourishment, but either passes off by the bowels before it produces some other humour, crude and corrupted; or, if it continue for a longer time, produces suffering or disease hard to cure, destroying at the same time the natural nourishment, or even the flesh itself which needs nourishment. But even though it be expelled at length, overcome by certain medicines, or by better food, or by the natural forces, it is not got rid of without doing much harm, since it bears no peaceful aspect towards what is natural, because it cannot coalesce with nature.