Universa Philosophia

The definition is converted simply with the defined, as animal rationale with man, and this animal rationale with this man. Likewise a proposition with a singular subject and predicate, as Petrus est hoc album, ergo hoc album est Petrus. You ask, how is this Petrus est homo converted? For it seems to be converted per accidens, as ergo aliquis homo est Petrus, where the quantity of the proposition is changed. Someone will answer that every proposition in which a universal is affirmed of its inferior is converted simply, because the universal is then predicated confusedly: for this Petrus est homo is equivalent to this, Petrus est aliquis homo, because homo is said confusedly of Peter: which proposition is of the same quantity as this, Petrus est homo. On the contrary, because this proposition omnis homo est animal is converted per accidens thus, aliquod animal est homo, therefore similarly the proposition of the argument, because omnis homo est animal is equivalent to this, est aliquod animal. The reason is that the quantity of a proposition is taken from the subject, but the subject of this, Petrus est homo, is of a different quantity from this aliquis homo est Petrus, because the one is singular, the other particular confused; whence that proposition is converted per accidens.