AENESIDEMUS of Knossos (Αινεσιδημος) — a Skeptic philosopher, one of the most important representatives of early Pyrrhonism in the first century BC.

Most historians think that Aenesidemus came from Knossos in Crete, but some (e.g., Photius, who passed on the content of Aenesidemus' major work) describe him as Aenesidemus of Aigai (a name belonging to several places in Greece). No dates can be assigned to his birth and death. Since he is not mentioned in any of Cicero's works, it is thought that he became known only after his death (43 BC). He spent most of his life in Alexandria where he taught and founded a Skeptical school, and his major work was written for the needs of the school. As a result of Aenesidemus' teaching, a strong Skeptical current arose in Alexandria and persisted until the late Roman Empire.

Aenesidemus' chief work was made up of a treatise in eight books: Πυρρωνειοι λογοι [Pyrróneioi logoi] (Pyrrhonian thoughts or Pyrrhonian statements). This work did not survive, but we know its content indirectly (primarily in condensed form in the Bibliotheca of Photius). Other works by Aenesidemus have also perished. Only the titles of the most important works are known: Περι σοφιας [Perí sophías] (On wisdom); Περι ζητησεως [Perí dzetéseos] (On the search [for truth])]; Κατα σοφιας [Katá sophías] (Against wisdom); Υποτυπωσι&sigmaf εις τα πυρρωνεια [Hypotýposis eis ta pyrróneia] (Outline of Pyrrhonism).

Aenesidemus fought against the dogmatic philosophy cultivated in the Academy. He contributed to the restoration of Skeptical doctrine in the first century BC. He thought the right way for a man to live is to abstain from all judgments, whether negative or positive. This attitude would lead to undisturbed calm (ataraxia) and this would bring happiness.

In this theory he accented the relative character of human perceptions. He stated that men are not able to know the "truth of things" either by the senses or by any method of reasoning.

Aenesidemus' most important philosophical accomplishment is thought to be his collection and systematization of Skeptical arguments that deny that things can be known by the senses. These arguments are presented in ten tropes. 1) A man perceives in a different way than does a beast, for his senses are constructed differently. We cannot regard the human mode of perception as the best, since we know, for example, that the hawk has better eyesight, and the dog has a better olfactory sense. 2) Men differ one from the other not only in appearance but also in character. What is harmful to one is beneficial to another. What seems good to one will seem the opposite to another. No one can decide which man is right. 3) We know things by the different senses. An aromatic oil is pleasant to the olfactory sense, but has an unpleasant taste. It is hard to distinguish which of the senses better presents the nature of the thing. 4) Perceptions depend upon circumstances. We see one way when we are healthy and in another way when we are sick; one way when we are in motion, another way when at rest; one way when we love, another way when we hate. 5) Our perceptions are also influenced by the conditions in which we perform our observations. An oar appears straight in the air, but appears bent when we put it in the water. 6) We never know things in separation, but always in connection with something else. Since no thing occurs in pure form, we do not know the thing's particular individual properties. 7) Things possess different properties depending on the quantity in which they occur. For example, a medicine becomes harmful when it is used in excess. 8) We always perceive things from a certain point of view, in view of something, and so our knowledge is always relative and never concerns the essence of things. 9) Things are also judged differently with respect to how frequently they occur. Things that occur more rarely are held to be more precious than things that occur in great quantities and are easy to obtain. For example, gold would lose its value if it could be obtained anywhere. 10) Upbringing, customs and beliefs have an essential influence on the human vision of the world, and for this reason there are essential differences of opinion among men as to what is true and false, good and evil.

Aenesidemus devoted much attention to a critique of the principle of causality. He presented eight arguments (tropes) to show that reasonings based on the principle of causality lead to errors.

Aenesidemus' "Heracliteanism" led to many disputes. These disputes are mentioned primarily in the writings of Sextus Empiricus. The presence of many Heraclitean views in Aenesidemus' thought has been explained in many ways. Some (e.g., Diels) thought that these views were wrongly ascribed to him because Sextus Empiricus made a mistake in his interpretation of sources. Others thought that Aenesidemus changed from Skepticism to dogmatism in his later views. Still others have pointed out that Heraclitean theses served Aenesidemus in the practical demonstration of his Skeptical position. Some have objected that when Aenesidemus cited Heraclitus' dogmatic philosophy, he was demonstrating indirectly the superiority of dogmatic (positive) philosophy over Skepticism, since if there were no dogmatic philosophical systems such as that of Heraclitus, the Skeptic would not have anything to call into doubt.

The adherents of Skepticism in the Roman Empire drew on the views of Aenesidemus. Sextus Empiricus in particular in his Outlines of Pyrrhonism and in his treatise, Against the Dogmatists based his thought on that of Aenesidemus.

A. Krokowiecz, Scepticyzm grecka. Od Pyrrona do Karneadesa [Greek Skepticism. from Pyrrho to Carneades], Wwa 1964, 88-92, 114-204; I. Dąmbska, Słownik filozofów [Dictionary of philosophers], Wwa 1966, 4-5; A. Krokiewicz, Zarys filozofii greckiej. Od Talesa do Platona [Outline of Greek philosophy. From Thales to Plato], Wwa 1971, 2000, 414-431; J. Legowicz, Historia filozofii starożytnej Grecji i Rzymu [History of ancient philosophy of Greece and Rome] Wwa 1973, 19862, 292-295; Reale IV 173-208.

Robert T. Ptaszek

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