Philosophers who hold no doctrine and suspend judgement on everything. The label σκεπτικός‎‎ (‘inquirer’, but used with the implicit understanding that the inquiry does not end) was introduced in the 1st cent. bce, probably by the younger Pyrrhonists; before then, these philosophers would be known as Pyrrhonists or Academics, respectively.

Early Pyrrhonism. According to the ancient tradition, the founder of scepticism was Pyrrhon of Elis (c.365–275 bce). He held that it was not possible to determine whether things are one way rather than another, and that one should therefore refrain from asserting anything. Arguments to the effect that conflicts of appearances and opinions cannot be decided had been around since the time of Protagoras and Democritus, but Pyrrhon was probably the first to adopt the attitude of non-assertion and promote it as the foundation of peace of mind or tranquillity. Pyrrhon's school does not seem to have had any immediate followers after the time of his main pupil, Timon (2) of Phlius.

Scepticism in theAcademy. The second version of scepticism was developed in the Academy from Arcesilaus in the 3rd cent. bce to Philon (3) of Larissa in the first. The Academic sceptics saw themselves as followers of Socrates. They practised his dialectical method by arguing for and against any given thesis and refuting the doctrines of other philosophers. Arcesilaus' criticism of Stoic epistemology (see stoicism) started a lively debate between the two schools that went on for two centuries and covered all parts of philosophy, including theology and ethics. Both sides were forced to revise and refine their arguments in the process. While the Academics would argue that, given Stoic assumptions, knowledge was impossible, the Stoics would defend their conception of knowledge and force the Academics to elaborate a sophisticated defence of the claim that it is possible to lead a normal life while suspending judgement on everything. The most detailed reply to the ‘inactivity’ argument, Carneades' theory of the ‘plausible impression’, eventually became the basis of a return to philosophical doctrine in the Academy under the scholarchate of Philon of Larissa, in the form of the cautious fallibilism presented in Cicero's Academici libri.

The Pyrrhonist Revival. In the 1st cent. bce, when Academic scepticism had lost its vigour, Pyrrhonism was revived by Aenesidemus as a more radical form of scepticism. This new version of Pyrrhonism was clearly influenced by the preceding debate between Academics and Stoics. But Aenesidemus also picked up the older arguments from conflicting appearances, systematizing them in his list of ten ‘Modes’ for inducing suspension of judgement, and he revived the claim that scepticism can serve as a way to tranquillity. Unlike Pyrrhon, the later Pyrrhonists would avoid any dogmatic assertions about the impossibility of knowledge, presenting their views instead as an expression of what appeared to them to be the case. They also worked out a reply to the charge that scepticism makes life impossible, by saying that they would be guided in their everyday activities by simple appearances without strong assent.

Aenesidemus' main source of information about Pyrrhon was probably Timon, but it is very likely that he was also influenced by the epistemological and methodological debates that had been going on between the different schools of medicine (see medicine, § 5.3). The list of Aenesidemus' followers contains the names of several prominent doctors, and the final and most sophisticated version of Pyrrhonism is preserved for us in the writings of the Empiricist physician Sextus Empiricus (fl. c.ce 200). After Sextus, we do not hear of any important representatives of the school, and scepticism as a philosophical movement seems to have come to an end.