Sextus Empiricus gives us (adv. Math. VII. 155, 151-153) a more particular explanation of this philosophy, which is preserved to us only as being in opposition to the Stoics. Arcesilaus asserted as against the Stoics, that everything is incomprehensible (ἀκατάληπτα). He thus combated the conception of thought (καταληπτικὴν φαντασίαν), which to the Stoics is the point of most importance and the concrete truth. Arcesilaus further attacked the Stoics thus: “They themselves say that the conception of thought is the mean between scientific knowledge and opinion, the one of which pertains alone to fools and the other alone to wise men; the conception of thought is common to both, and the criterion of the truth. Arcesilaus here argued in such a way as to show that between scientific knowledge and opinion the conception of thought is no criterion, for it is either in the wise man or the fool, and in the former it is knowledge, and in the latter, opinion. If it is nothing excepting these, there remains to it nothing but an empty name.” For knowledge must be a developed consciousness derived from reasons, but these reasons, as conceptions of thought, Arcesilaus states to be just such thoughts as those of the fool. They are thus, no doubt, the concrete directing power which constitutes the principal content of our consciousness; but it is not proved that they are the truth. Thus this mean, as judging between reason and opinion, pertains equally to the wise man and the fool, and may be error or truth equally; and thus the wise man and the fool have the same criterion, and yet they must, in relation to the truth, be distinguished from one another.