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Aenesidemus, Greek philosopher, was born at Cnossus in Crete and taught at Alexandria, probably during the first century BC.

He was the leader of what is sometimes known as the third sceptic 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. Of these the two former are important. The reasons for doubt are given in the form of the ten "tropes":

1. 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 objects are known only indirectly through the medium of air, moisture, &c.,
7. 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
10. all men are brought up with different beliefs, under different laws and social conditions.

Truth varies infinitely under circumstances whose relative weight cannot be accurately gauged. There is, therefore, no absolute knowledge, for every man has different perceptions, and, further, arranges and groups his data in methods peculiar to himself; so that the sum total is a quantity with a purely subjective validity. The second part of his work consists in the attack upon the theory of causality, in which he adduces almost entirely those considerations which are the basis of modern scepticism. Cause has no existence apart from the mind which perceives; its validity is ideal, or in modern terminology, subjective. 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, since effect cannot precede cause, cause must precede effect, and there must be an instant when cause is not effective, that is, is not itself. By these and similar arguments he arrives at the fundamental principle of Scepticism, the radical and universal opposition of causes; iravrl hbyy Xoyos dw/ceiTcu.

Having reached this conclusion, he was able to assimilate the physical theory of Heraclitus, as is explained in the Hypoiyposes of Sextus Empiricus. For admitting that contraries co-exist for the perceiving subject, he was able to assert the co-existence of contrary qualities in the same object. Having thus disposed of the ideas of truth and causality, he proceeds to undermine the ethical criterion, and denies that any man can aim at Good, Pleasure or Happiness as an absolute, concrete ideal. All actions are product of pleasure and pain, good and evil. The end of ethical endeavour is the conclusion that all endeavour is vain and illogical. The main tendency of this destructive scepticism is essentially the same from its first crystallization by Aenesidemus down to the most advanced sceptics of today. See also Carneades and Arcesilaus. Of the Ilvpp&veioi. Aoyoi nothing remains; we have, however, an analysis in the Myriobiblion of Photius.

• See Zeller's History of Greek Philosophy, Émile Saisset, Ænesideme, Pascal, Kant; Ritter and Preller, §§ 364-370.

This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.

(See Philosopedia with statement by Bertrand Russell [[1]])