a species of polygons. This classification, though it is of high value in the clearing up of our conceptions of the essential contrasted with the accidental, the relation of the genus, differentia and definition and so forth, is of more significance in connection with abstract sciences, especially mathematics, than for the physical sciences. It is superior on the whole to the Porphyrian scheme, which has grave defects. As has been said, the Porphyrian scheme classifies universals as predicates of individuals and thus involves the difficulties which gave rise to the controversy between realism and nominalism. How are we to distinguish species from the genus? Napoleon was a Frenchman, a man, an animal. In the second place, how do we distinguish property and accident? Many so-called accidents are predicable necessarily of any particular persons. This difficulty gave rise to the distinction of separable and inseparable accidents, which is one of considerable difficulty. Some Aristotelian examples may be briefly mentioned. In the true statement “Man is a rational animal,” the predicate is convertible with the subject and states its essence; therefore, “rational animal” is the definition of a man. The statements “Man is an animal” and “Man is rational,” while true, are not convertible; their predicate terms, however, are parts of the definition and hence are the genus and differentia of man. On the other hand, the statement “Man is capable of learning grammar” is true and convertible; but “capable of learning grammar” does not state the essence of man and is, therefore, a property of man. The true statement “Man is featherless” offers an example of an accident. Its predicate is not convertible with its subject, nor is it part of the definition; accordingly, it expresses only an accidental characteristic of man. Notes References Term logic Concepts in logic