The second portion of Plato's theory, advanced to explain what virtue is, presents nothing more satisfactory. Virtue is useful or profitable: but neither health, strength, beauty, wealth, power, &c., are profitable, unless rightly used: nor are justice, moderation, courage, quick apprehension, good memory, &c., profitable, unless they are accompanied and guided by knowledge or prudence.[35] Now if by _profitable_ we have reference not to the individual agent alone, but to other persons concerned also, the proposition is true, but not instructive or distinct. For what is meant by _right use_? To what ends are the gifts here enumerated to be turned, in order to constitute right use? What again is meant by _knowledge_? knowledge of what?[36] This is a question put by Sokrates in many other dialogues, and necessary to be put here also. Moreover, knowledge is a term which requires to be determined, not merely to some assignable object, but also in its general import, no less than virtue. We shall come presently to an elaborate dialogue (Theætêtus) in which Plato makes many attempts to determine knowledge generally, but ends in a confessed failure. Knowledge must be knowledge _possessed by some one_, and must be knowledge of _something_. What is it, that a man must know, in order that his justice or courage may become profitable? Is it pleasures and pains, with their causes, and the comparative magnitude of each (as Sokrates declares in the Protagoras), in order that he may contribute to diminish the sum of pains, increase that of pleasures, to himself or to the society? If this be what he is required to know, Plato should have said so--or if not, what else--in order that the requirement of knowledge might be made an intelligible condition. [Footnote 35: Plato, Menon, pp. 87-88.] [Footnote 36: See Republic, vi. p. 505 B, where this question is put, but not answered, respecting [Greek: phro/nêsis].] [Side-note: Subject of Menon; same as that of the Protagoras--diversity of handling--Plato is not anxious to settle a question and get rid of it.]