That this thing may be understood, it must be clearly known that to be a man is one thing, and to be a pope or an emperor is another; just as to be a man is different from being a father or a ruler. A man is that which exists by its essential form, which gives it its genus and species, and by which it comes under the category of substance. But a father is that which exists by an accidental form, that is, one which stands in a certain relation which gives it a certain genus and species, and through which it comes under the category of relation. If this were not so, all things would come under the category of substance, seeing that no accidental form can exist by itself, without the support of an existing substance; and this is not so. Seeing, therefore, that the Pope and the Emperor are what they are by virtue of certain relations: for they owe their existence to the Papacy and the Empire, which are both relations, one coming within the sphere of fatherhood, and the other within that of rule; it manifestly follows that both the Pope and the Emperor, in so far as they are Pope and Emperor, must come under the category of relation; and therefore that they must be brought under some head of that genus. I say then that there is one standard under which they are to be brought, as men; and another under which they come, as Pope and Emperor. For in so far as they are men, they have to be brought under the best man, whoever he be, who is the measure and the ideal of all mankind; under him, that is, who is most one in his kind,[304] as may be gathered from the last book to Nicomachus.[305] When, however, two things are relative, it is evident that they must either be reciprocally brought under each other, if they are alternately superior, or if by the nature of their relation they belong to connected species; or else they must be brought under some third thing, as their common unity. But the first of these suppositions is impossible: for then both would be predicable of both, which cannot be. We cannot say that the Emperor is the Pope, or the Pope the Emperor. Nor again can it be said that they are connected in species, for the idea of the Pope is quite other than the idea of the Emperor, in so far as they are Pope and Emperor. Therefore they must be reduced to some single thing above them. [Footnote 304: "_Ad existentem maxime unum in genere suo._"] [Footnote 305: _Eth._ x. 5, 7.--(W.)]