Skepticism doubts whether certain knowledge of the world, the self or moral truth is available. In its ancient form it treats that doubt as a route to tranquility, not as a crisis.
Pyrrho founded the practice and Sextus Empiricus recorded it. Pyrrhonian skepticism worked by setting opposing arguments of equal weight against each other until judgment was suspended, on the theory that the suspension itself, epoché, produced the peace of mind that dogmatic certainty had promised and failed to deliver. This is a way of living and not a set of conclusions. It differs from the Academic skepticism that held knowledge to be impossible, a claim Pyrrhonians pointed out is itself a dogma.
Montaigne revived the tradition in the sixteenth century, taking "what do I know?" as his motto amid the French wars of religion, and the timing matters. Skepticism's political payoff is toleration. If no one can be certain enough to justify burning a neighbor over doctrine, persecution loses its warrant, and much of the early case for religious liberty runs through exactly that argument. Descartes then turned doubt into a method for defeating itself. Hume produced the most influential modern version: mitigated, cheerful, and content to note that nature obliges us to believe what reason cannot establish.
Its political descendants appear in every tradition that distrusts certainty in power, among them Popper's fallibilism, Berlin's pluralism and Shklar's liberalism of fear. The standard objection is self-refutation, that the skeptic must exempt their own position. The Pyrrhonian reply is that skepticism reports a state and does not assert a thesis, which has never fully satisfied its critics or fully collapsed under them.