AENESIDEMUS, Greek philosopher, was born at Cnossus, in Crete and taught at Alexandria, probably during the I st century B.C. He was the leader of what is sometimes known as the third sceptical school and revived to a great extent the doctrine of Pyrrho and Timon. His chief work was the Pyrrhonian Principles addressed to Lucius Tubero. His philosophy consisted of four main parts : the reasons for scepticism and doubt, the attack on causality and truth, a physical theory and a theory of morality. The reasons for doubt are given in the form of the ten "tropes": (I) different animals manifest different modes of perception ; (2) similar differences are seen among individual men; (3) even for the same man, sense-given data are self-contradictory; (4) vary from time to time with physical changes, and (5) according to local relations; (6) and (7) objects are known only indirectly through the medium of air, moisture, etc., and are in a condition of perpetual change in colour, temperature, size and motion ; (8) all perceptions are relative and interact one upon another; (9) our impressions become less deep by repetition and custom; and (i o) all men are brought up with different beliefs, under different laws and social conditions. Truth varies infinitely under circum stances whose relative weight cannot be accurately gauged. There is, therefore, no absolute knowledge, for every man has different perceptions, and groups his data in methods peculiar to himself. In attacking causality he adduces almost entirely those considera tions which are the basis of modern scepticism. Cause has no existence apart from the mind which perceives ; its validity is ideal. The relation between cause and effect is unthinkable. If the two things are different, they are either simultaneous or in succession. If simultaneous, cause is effect and effect cause. If not, cause must precede effect, and there must be an instant when cause is not effective, that is, is not itself. Thus he arrives at the fundamental principle of scepticism, the universal opposition of causes: ravri. XOsycp XO-yos Cu/riserat (see SCEPTICISM).
For the immediate successors of Aenesidemus see AGRIPPA, SEXTUS