Aenesidēmus in Harpers Dictionary of Classical Antiquities
（Αἰνησίδημος). A skeptic, born at Cnossus, in Crete, who lived a little later than the time of Cicero. He wrote eight books on the doctrines of Pyrrho (q.v.), of which extracts may be found in Photius, Cod. 212.
Aenesidemus in Wikipedia
Aenesidemus (Ancient Greek: Αἰνησίδημος, Ainêsidemos) was a Greek sceptical philosopher, born in Knossos on the island of Crete. He lived in the first century BC, taught in Alexandria and flourished shortly after the life of Cicero. He was probably a member of Plato's Academy, but due to his rejection of their theories he revived the principle of epoché (εποχή), or suspended judgement, originally proposed by Pyrrho and Timon, as a solution to what he considered to be the insoluble problems of epistemology. His school is most commonly referred to as Pyrrhonism, but also as the third sceptic school. His chief work, the Pyrrhoneia (Πυρρώνειοι λóγοι) discussed four main ideas: the reasons for scepticism and doubt, arguments against causality and truth, a physical theory and an ethical theory. Of these, the former are the most significant and his reasons for the suspension of judgment were organized into ten "tropes", or modes. Very little is known about him as none of his works have survived, though he has been mentioned and discussed in detail by Photius (in his Myriobiblion) and Sextus Empiricus, and also to a lesser extent by Diogenes Laertius and Philo of Alexandria.
There is no definite evidence about the life of Aenesidemus, but his most important work, the Pyrrhoneia was known to be dedicated to Lucius Tubero, a friend of Cicero and member of Plato's Academy. Based on this information, scholars have assumed the Aenesidemus himself was also a member of the Academy. Furthermore it has been assumed that he took part under the leadership of Philo of Larissa and probably developed his sceptical philosophy in reaction to Philo's fallibilism.
His chief work, known in Ancient Greek as Pyrrhôneoi logoi (Πυρρώνειοι λóγοι) and often rendered into English as the "Pyrrhonian Discourses" or "Pyrrhonian Principles", dealt primarily with man's need to suspend judgment due to our epistemological limitations. It was divided into eight books, but it has not survived. This was developed in response to the philosophical dogmatism in Plato's Academy at the time. Aenesidemus argued that his contemporaries in the Academy were unjustified in strongly affirming some theories, including ideas of the Stoics, while firmly denying others. He argued that one should "determine nothing", meaning that nothing should be either affirmed or denied. Stated more formally, Aenesidemus would never assert the statements "X is always F" or "X is never F", but rather only the statements "it is not the case that X is always F" or "it is not the case the X is never F". Given this, it seems that under his form of scepticism the only acceptable statements are negative ones. He bases this on his argument that holds that humans are incapable of knowing anything more, which is fomalised in the "ten tropes".
The Ten Tropes
The reasons for epoché are given in what are often called the ten tropes or ten modes. The argument is as follows:
1. Different animals manifest different modes of perception;
2. Similar differences are seen among individual men;
3. For the same man, information perceived with the senses is self-contradictory
4. Furthermore it varies from time to time with physical changes
5. In addition, this data differs according to local relations
6. Objects are known only indirectly through the medium of air, moisture, etc.
7. These objects 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 critical through repetition and custom
10. All men are brought up with different beliefs, under different laws and social conditions
In other words, he argues that truth varies infinitely under circumstances whose importance to one another cannot be accurately judged by human observers. He therefore rejects any concept of absolute knowledge, since every man has different perceptions, and he arranges this sense-gathered data in methods peculiar to himself.
Arguments against causality
The second part of his work attacks the theory of causality. The proofs in his arguments bear strong resemblance to the precepts of modern skepticism. For example, he argues that cause has no inherent existence, but rather it only exists within a perceiving mind, and as such its validity can only be ideal, or subjective. To a human mind, the true relation between a cause and effect is impossible to determine. He argues that if cause and effect are different, then they must occur either simultaneously or in succession. If they happen at the same time, then in essence cause is effect and effect is cause. If they are instead in succession, then the cause must precede the effect as an effect cannot precede its cause, and therefore there must be a space of time when the cause is actually not effective, meaning that it is essentially not itself. Through such arguments he arrives at the fundamental principle of scepticism: the radical and universal opposition of causes summed up in the phrase panti logo logos antikeitai ("to every argument another argument is opposed with the same strength").
Physical and ethical theories
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.