Gleanings in Dialectics

...if it were converted into itself per accidens, it would not be a simple conversion. Because the supposition of the terms is changed, as if I said "Every man is risible," I would say afterwards "Every risible thing is a man," the term "risible" does not suppose in the same way in both. Instead, in the first, it is only confused; in the latter, it is distributively movable. And his error indeed proceeded from there, because he was ignorant of the definition of simple conversions. And according to him, we have to say that no universal affirmative can be simply converted. For the predicate term would always suppose in a different way, nor could any particular negative be simply converted in any matter for the same reason. Yet Boethius says the opposite in the treatise he makes on the categorical and hypothetical proposition. Furthermore, propositions about the necessary would not be converted simply like those about existence, which is against Aristotle in the Prior Analytics. For in these that are about the necessary, the same supposition is not preserved when conversion is made. They are not said to be simply converted, the universal affirmative or the particular negative, because they are not converted in every matter. And to speak summarily, just as Aristotle in the first book of the Prior Analytics rejects the forms of syllogisms in which sometimes the true is held, sometimes not, according to the diversity of the matter, so we shall say that those propositions are simply converted to which such conversion belongs in every matter, but not simply to those to which it belongs in some and not in others. And I consider this to have been said quite reasonably, since the whole intention of convertible propositions is reduced to syllogisms, wherefore it will be the same reason in both cases. From the aforementioned, it is clear that this consequence is not valid for these two propositions: the terms are the same, and they are transposed, and they suppose in the same way, therefore one is said to be converted into the other simply. Just as if we said, "Every man is an animal," "Only an animal is a man." Although the terms are transposed and suppose in the same way, yet one is not said to be converted into the other, from the fact that it is not the same determination.