Although Greek scepticism centred on the value of enquiry and questioning, scepticism is now the denial that knowledge or even rational belief is possible, either about some specific subject-matter (e.g. ethics) or in any area whatsoever. Classically, scepticism springs from the observation that the best methods in some area seem to fall short of giving us contact with the truth (e.g. there is a gulf between appearance and reality), and it frequently cites the conflicting judgements that our methods deliver, with the result that questions of truth become undecidable. In classical thought the various examples of this conflict were systematized in the ten tropes of Aenesidemus. The scepticism of Pyrrho and the new Academy was a system of argument and indeed ethics opposed to dogmatism, and particularly to the philosophical system-building of the Stoics. As it has come down to us, particularly in the writings of Sextus Empiricus, its method was typically to cite reasons for finding an issue undecidable (sceptics devoted particular energy to undermining the Stoic conception of some truths as delivered by direct apprehension or katalepsis). As a result the sceptic counsels epochē or the suspension of belief, and then goes on to celebrate a way of life whose object was ataraxia, or the tranquillity resulting from such suspension of belief. The process is frequently mocked, for instance in the stories recounted by Diogenes Laertius that Pyrrho had to be restrained from sublimely walking over precipices, leaving stuck people in bogs, and so on, since his method denied him confidence that there existed the precipice or the bog (the legends may have arisen from a misunderstanding of Aristotle, Metaphysics G, iv. 1008 b where Aristotle argues that since sceptics don’t do such things, they actually accept the doctrines they pretend to reject). In fact ancient sceptics allowed confidence in ‘phenomena’, reserving their scepticism for more theoretical confidences, but quite how much fell under the heading of phenomena is not always clear.

Sceptical tendencies emerged in the 14th-century writings of Nicholas of Autrecourt (fl. 1340). His criticisms of any certainty beyond the immediate deliverance of the senses and basic logic, and in particular of any knowledge of either intellectual or material substances, anticipate the later scepticism of Bayle and Hume. The latter distinguishes between Pyrrhonistic or excessive scepticism, which he regarded as unliveable, and the more mitigated scepticism which accepts everyday or common-sense beliefs (albeit not as the delivery of reason, but as due more to custom and habit), but is duly wary of the power of reason to give us much more. Mitigated scepticism is thus closer to the attitude fostered by ancient sceptics from Pyrrho through to Sextus Empiricus. Although the phrase ‘Cartesian scepticism’ is sometimes used, Descartes himself was not a sceptic, but in the method of doubt uses a sceptical scenario in order to begin the process of finding a secure mark of knowledge. Descartes himself trusts a category of ‘clear and distinct’ ideas, not far removed from the phantasia kataleptikē of the Stoics.

Scepticism should not be confused with relativism, which is a doctrine about the nature of truth, and may be motivated by trying to avoid scepticism. Nor is it identical with eliminativism, which counsels abandoning an area of thought altogether, not because we cannot know the truth, but because there are no truths capable of being framed in the terms we use.