The categories are exhaustive, but do they fulfil another requisite of a good division--are they mutually exclusive? Aristotle himself raised this question, and some of his answers to difficulties are instructive. Particularly his discussion of the distinction between Second Substances or Essences and Qualities. Here he approximates to the modern doctrine of the distinction between Substance and Attribute as set forth in our quotation from Mansel at p. 110. Aristotle's Second Essences ([Greek: deuterai ousiai]) are common nouns or general names, Species and Genera, _man_, _horse_, _animal_, as distinguished from Singular names, _this man_, _this horse_, which he calls First Substances ([Greek: protai ousiai]), essences _par excellence_, to which real existence in the highest sense is attributed. Common nouns are put in the First Category because they are predicated in answer to the question, What is this? But he raises the difficulty whether they may not rather be regarded as being in the Third Category, that of Quality ([Greek: to poion]). When we say, "This is a man," do we not declare what sort of a thing he is? do we not declare his Quality? If Aristotle had gone farther along this line, he would have arrived at the modern point of view that a man is a man in virtue of his possessing certain attributes, that general names are applied in virtue of their connotation. This would have been to make the line of distinction between the First Category and the Third pass between First Essence and Second, ranking the Second Essences with Qualities. But Aristotle did not get out of the difficulty in this way. He solved it by falling back on the differences in common speech. "Man" does not signify the quality simply, as "whiteness" does. "Whiteness" signifies nothing but the quality. That is to say, there is no separate name in common speech for the common attributes of man. His further obscure remark that general names "define quality round essence" [Greek: peri ousian], inasmuch as they signify what sort a certain essence is, and that genera make this definition more widely than species, bore fruit in the mediaeval discussions between Realists and Nominalists by which the signification of general names was cleared up.