 Socrates then introduces the issue of knowledge, which Critias agrees is necessary to the exercise of sophrosune. But knowledge of what? Critias suggests that it is self-knowledge that is sophrosune (164d). Critias has in mind the kind of self-knowledge that consists in knowing what one knows and what one does not know. From this it is a short step to defining the virtue as the knowledge of itself and the other kinds of knowledge—i.e., the knowledge that enables one to evaluate all sorts of knowledge claims (166e). Socrates raises two questions about this kind of knowledge: whether it is possible, and whether it is beneficial (relevant because the virtue was assumed to be beneficial). It would seem impossible for there to be such a kind of knowledge, since all other examples of kinds of knowledge have subject matter specific to each, and this would be an exception. To know whether a candidate has knowledge of medicine, for example, one would need to know medicine. As for the question of benefit, it would appear that such knowledge is beneficial (173a, ff.), because it would allow us to submit all decisions to knowledge. But the appearance is deceptive, because the knowledge of knowledge does not add anything to the various kinds of knowledge (such as medicine) on which we depend.