Aenesidemus (1st c. CE.)

Aenesidemus was the founder of Pyrrhonian Skepticism. He was born at
Gnossus in Crete, but lived at Alexandria and flourished shortly after
Cicero. Aenesidemus originally was a member of Plato's Academy. From
the time of Arcesilaus through Carneades, at least, the Academy was
skeptical. By the time of Aenesidemus, however, the Academy had
splintered into several competing factions and considerably softened
or even abandoned its skepticism, as a result of its dialectical
interchange with the Stoics. One head of the Academy, Philo, turned to
a form of moderate fallibilism, in which one could assent to many
beliefs and gain knowledge, although not certainty, while a later
head, Antiochus, propounded a dogmatic and syncretistic philosophy,
claiming that at bottom Plato, the Stoics, and many other philosophers
were really saying the same thing.

Aenesidemus complained that the situation had deteriorated to the
point where the Academics were no more than "Stoics in conflict with
Stoics," and he broke with the Academy and founded his own school,
taking Pyrrho as its namesake. To strengthen the cause of skepticism,
he developed the ten tropes or modes of skepticism—a set of skeptical
argument forms, or modes, to show that judgment must be withheld on
any issue. All are based on some form of relativity—e.g., the same
object can give rise to different perceptions, depending on the bodily
condition of the percipient–conjoined with the claim that there is no
criterion by which to adjudicate which of the perceptions, customs,
etc., are correct. Although Diogenes Laertius attributes the ten modes
to Pyrrho, it is likely that they owe their existence to Aenesidemus.
Extracts of the ten modes are found in Photius.

Briefly, the ten modes are as follows: (1) The feelings and
perceptions of all living beings differ. (2) People have physical and
mental differences, which make things appear different to them. (3)
The different senses give different impressions of things. (4) Our
perceptions depend on our physical and intellectual conditions at the
time of perception. (5) Things appear different in different
positions, and at different distances. (6) Perception is never direct,
but always through a medium. For example, we see things through the
air. (7) Things appear different according to variations in their
quantity, color, motion, and temperature. (8) A thing impresses us
differently when it is familiar and when it is unfamiliar. (9) All
supposed knowledge is predication. All predicates give us only the
relation of things to other things or to ourselves; they never tell us
what the thing in itself is. (10) The opinions and customs of people
are different in different countries.