[xii][1] Their argument based on reason runs as follows. Adopting a principle from the tenth book of the Metaphysics, they say: all things belonging to a single species are referred to one thing which is the measure for all things which belong to that species; but all men belong to the same species; therefore they are to be referred to one man as their common measure.

[2] And since the supreme Pontiff and the Emperor are men, if that conclusion is valid, it must be possible to refer them to a single man. And since the Pope must not be referred to any other man, it remains that the Emperor along with all other men must be referred to him, as to their measure and rule; from this too the conclusion they want to reach does indeed follow.

[3] To refute this argument I say that, when they say ‘Those things which are of one species must be referred to a single thing of that species which is the measure for the species’, they are correct. And similarly they are correct when they say that all men belong to a single species; and again they reach a correct conclusion when from these premisses they infer that all men are to be referred to a single measure for the species. But when from this conclusion they draw their inference concerning the Pope and the Emperor, they commit the accidental fallacy.

[4] To clarify this it must be borne in mind that it is one thing to be a man, another to be Pope; and in the same way it is one thing to be a man and another to be Emperor, just as it is one thing to be a man and another to be a father and a master.

[5] For man is what he is because of his substantial form, by virtue of which he belongs to a species and a genus and is placed in the category ‘substance’; whereas a father is what he is because of his accidental form, which consists of a relationship by virtue of which he belongs to a certain species and genus and comes into the category of ‘being related to’, that is to say ‘relationship’. If this were not so, everything would fall within the category ‘substance’, inasmuch as no accidental form can have autonomous existence without being located in an existing substance; and this is false.

[6] Since therefore Pope and Emperor are what they are by virtue of certain relationships, i.e. by virtue of Papal and Imperial office, which are respectively relationships of ‘paternity’ and of ‘lordship’, it is clear that Pope and Emperor must be assigned as Pope and Emperor to the category of relationship, and as a consequence be referred to something within that category.

[7] So I am saying that there is one measure to which they are to be referred as men, and another as Pope and Emperor. For as men they are to be referred to the perfect man, who is the measure of all the others, and the model, as it were – whoever he might be – of what is most unified in his species, as we can deduce from the end of the Ethics.

[8] Insofar as they are terms which express a relationship, as is obvious, they are either to be referred one to the other (if one is subordinate to the other, or if they are related to one another within the species by the type of relationship), or else to some third entity to which they are to be referred as to a common unity.

[9] But it cannot be maintained that one is subordinated to the other, because if this were the case one would be predicated of the other; and this is false, for we do not say ‘the Emperor is Pope’, nor vice versa. Nor can it be said that they are related to one another within the species, for the Pope’s function is one thing and the Emperor’s another, precisely because they are Pope and Emperor; therefore they are to be referred to some other thing in which they find their unity.

[10] Consequently it must be grasped that as relationship stands to relationship, so the terms of relationship stand to one another. If therefore Papal and Imperial office, being relationships of authority, are to be referred to the principle of authority, from which they derive with their differentiating characteristics, then Pope and Emperor (being the terms of relationship) will be referable to some entity in which it is possible to discern that principle of authority without the other differentiating characteristics.

[11] And this will either be God himself, in whom all principles form an absolute unity, or else some entity lower than God, in which the principle of authority, derived from the absolute principle and differentiating itself from it, becomes distinctive and individual.

[12] Thus it is evident that Pope and Emperor, considered as men, are to be referred to one thing; but as Pope and Emperor they are to be referred to another; and thus the answer to their argument from reason is clear.