Again in the case where it is true that "all _x_s are _y_s," but not true that all "_y_s are _x_s," _y_ may be part of the definition of _x_ or it may not. If it is part of the definition of _x_ it will be either (3) a genus or wider class of which _x_ forms a subdivision, as when I say, "All men are animals," or (4) a difference, that is, one of the distinctive marks by which the _x_s are distinguished from other sub-classes or species of the same genus, as when I say, "All men are capable of discourse." Or finally (5) _y_ may be no part of the definition of _x_, but a characteristic which belongs both to the _x_s and some things other than _x_s. The predicate is then called an Accident. We have now exhausted all the possible cases, and may say that the predicate of a universal affirmative proposition is always either a definition, a proprium, a genus, a difference, or an accident. This classification reached the Middle Ages not in the precise form in which it is given by Aristotle, but with modifications mainly due to the Neo-Platonic philosopher Porphyry. In its modified form it is regarded as a classification of terms generally. Definition disappears from the list, as the definition is regarded as a complex made up of the genus, or next highest class to which the class to be defined belongs, and the differences which mark off this particular species or sub-class. The species itself which figures as the subject-term in a definition is added, and thus the "Five Words" of mediæval logic are enumerated as genus, species, difference, proprium, accident.