We have, then, it seems, arrived at the subject of Nutrition, which is the third and remaining consideration which we proposed at the outset. For, when the matter which flows to each part of the body in the form of nutriment is being worked up into it, this activity is _nutrition_, and its cause is the _nutritive faculty_. Of course, the kind of activity here involved is also an _alteration_, but not an alteration like that occurring at the stage of _genesis_.[53] For in the latter case something comes into existence which did not exist previously, while in nutrition the inflowing material becomes assimilated to that which has already come into existence. Therefore, the former kind of alteration has with reason been termed _genesis_, and the latter, _assimilation_. VIII Kai toinyn ho logos hêkein eoiken ho peri tês threpseôs, hos dê loipos esti kai tritos hôn ex archês prouthemetha. tou gar epirrheontos en eidei trophês panti || moriô tou trephomenou sômatos prosplattomenou 19 threpsis men hê energeia, threptikê de dynamis hê aitia. alloiôsis men dê kantautha to genos tês energeias, all' ouch hoiaper hê en tê genesei. ekei men gar ouk on proteron hysteron egeneto, kata de tên threpsin tô êdê gegonoti synexomoioutai to epirrheon kai dia tout' eulogôs ekeinên men tên alloiôsin genesin, tautên d' exomoiôsin ônomasan. IX Now, since the three faculties of Nature have been exhaustively dealt with, and the animal would appear not to need any others (being possessed of the means for growing, for attaining completion, and for maintaining itself as long a time as possible), this treatise might seem to be already complete, and to constitute an exposition of all the faculties of Nature. If, however, one considers that it has not yet touched upon any of _the parts_ of the animal (I mean the stomach, intestines, liver, and the like), and that it has not dealt with the faculties resident in these, it will seem as though merely a kind of introduction had been given to the practical parts of our teaching. For the whole matter is as follows: Genesis, growth, and nutrition are the first, and, so to say, the principal effects of Nature; similarly also the faculties which produce these effects--the first faculties--are three in number, and are the most dominating of all. But as has already been shown, these need the service both of each other, and of yet different faculties. Now, these which the faculties of generation and growth require have been stated. I shall now say what ones the nutritive faculty requires. IX