displayed by philosophy at this period. In other words, it is not against the reasoning processes that his criticisms are directed, but against the theory of causation on the objective side, and against the credibility of our immediate perceptions on the subjective side.[294] But, in both directions, he has worked out the difficulties of the old Sophists with a minuteness and a precision unknown to them; and some of his points have been found worth repeating in a different connexion by modern critics. Thus, in analysing the theory of causation, he draws attention to the plurality of causes as an obstacle to connecting any given consequent with one antecedent more than with another; to the illegitimate assumption that the laws inferred from experience hold good under unknown conditions; to the arbitrary assumption of hypothetical causes not evinced by experience; and to the absurdity of introducing a new difficulty for the purpose of explaining an old one.[295] With regard to causation itself, Aenesidêmus seems to have resolved it into action and reaction, thus eliminating the condition of antecedence and consequence, without which it becomes unintelligible.[296] The Alexandrian Sceptic’s general arguments against the possibility of knowledge resolve themselves into a criticism of what Sir W. Hamilton called Natural Realism, somewhat complicated and confused by a simultaneous attack on the theory of natural morality conceived as something eternal and immutable. They are summed up in the famous ten Tropes. Of these the first three are founded on the conflicting sensations produced by the same object when acting on different animals—as is inferred from the marked contrast presented by their several varieties of origin and structure,—on different men, and on the different senses of the same individual. The fourth, which has evidently an ethical bearing, enlarges on the changes in men’s views caused by mental and bodily changes, according to their health, age, disposition, and so forth. The next five Tropes relate to circumstances connected with the objects themselves: their distance and position as regards the spectator, the disturbance produced in their proper action by external influences such as air and light, together with the various membranes and humours composing the organs of sense through which they are apprehended; their quantitative variation, involving as it does opposite effects on the senses, or as with medicines, on the health; the law of relativity, according to which many things are only known when taken in company with others, such as double and half, right and left, whole and part; comparative frequency or rarity of occurrence, as with comets, which, while really of much less importance than the sun, excite much more interest from their being so seldom seen. Finally, the tenth Trope is purely ethical, and infers the non-existence of a fixed moral standard from the divergent and even opposite customs prevailing