Sextus Empiricus was a doctor of the Empiric school, and everything else about his life is conjecture. What he left is the only substantial surviving account of Pyrrhonian scepticism, written as a set of handbooks: the Outlines of Pyrrhonism, and a longer series against the dogmatists and the professors of the various disciplines.
The position he sets out is more disciplined than the modern word suggests. The Pyrrhonist does not assert that nothing can be known — that would itself be a dogma. He is someone who, having found the arguments on either side of every question to be of equal weight, suspends judgement, and then discovers that suspension is followed unexpectedly by tranquillity. The method is the systematic production of equipollence, and Sextus catalogues it: ten modes turning on the variability of appearances across animals, people, senses, circumstances and positions, and five later modes turning on the structure of justification itself — that every proof requires a further proof, that stopping arbitrarily is arbitrary, and that arguing in a circle proves nothing. That last set is the ancestor of every modern regress argument about the foundations of knowledge.
The sceptic still acts. He follows appearances, custom, the guidance of nature and the instruction of the arts, without believing that any of it reveals how things really are. Scepticism is presented as therapy rather than as a theory: the disturbance that makes people miserable comes from believing that things are good or bad by nature, and suspending that belief relieves it.
The Latin translation of 1562 is one of the genuinely detonating events in European intellectual history. Montaigne read it and built the Apology for Raymond Sebond around it, taking the motto What do I know. The revival of scepticism it caused is the problem Descartes set out to solve, and the method of doubt in the Meditations is Sextus's modes deployed in order to be defeated. Bayle turned the arguments into a dictionary that the Enlightenment quarried. Hume's account of causation and induction reworks the regress in a new key, and he adopted the Pyrrhonist's conclusion that nature relieves us of doubts reason cannot. Almost every modern discussion of what justifies belief still moves in the channels Sextus cut.