What little we know is from a description of his Pyrrhonian Discourses in the Myriobiblion of Photius from the 9th century, as well as a few mentions in the works of Sextus Empiricus, and to a lesser extent by Diogenes Laërtius.
There is, therefore, no absolute knowledge, for everyone has different perceptions, and, further, arranges and groups data in methods peculiar to themselves; so that the sum total is a quantity with a purely subjective validity.
By these and similar arguments he arrives at the fundamental principle of Scepticism, the radical and universal opposition of causes; παντὶ λόγῳ λόγος ἀντίκειται.
It is as follows (Ritter and Preller (1898) Historia Philosophiae Graecae section 41): "The natural philosopher is of opinion that what surrounds us is rational and endowed with consciousness.
For in sleep, when the openings of the senses close, the mind which is in us is cut off from contact with that which surrounds us, and only our connexion with it by means of respiration is preserved as a sort of root (from which the rest may spring again); and, when it is thus separated, it loses the power of memory that it had before.
Just, then, as embers, when they are brought near the fire, change and become red-hot, and go out when they are taken away from it again, so does the portion of the surrounding mind which sojourns in our body become irrational when it is cut off, and so does it become of like nature to the whole when contact is established through the greatest number of openings."
The true doctrine doubtless was, that sleep was produced by the encroachment of moist, dark exhalations from the water in the body, which cause the fire to burn low.
In a soul where the fire and water are evenly balanced, the equilibrium is restored in the morning by an equal advance of the bright exhalation.
"[6]His chief work, the Pyrrhonian Discourses (Ancient Greek: Πυρρώνειοι λóγοι, romanized: Pyrrhoneioi logoi) dealt primarily with man's need to suspend judgment due to our epistemological limitations.
"[7] In the 18th century, Gottlob Ernst Schulze wrote a book named after Aenesidemus in which he criticizes the defense of Immanuel Kant's Critique of Pure Reason made by Karl Leonhard Reinhold.