His most important work Outlines of Pyrrhonism (commonly abbreviated as PH) is a sort of summary of the Pyrrhonian sceptical doctrine (a kind of skepticism named after Pyrrho). The book is divided in three parts: Book I is a general introduction to the sceptical philosophy. The two other books are devoted to attacking the “dogmatic” systems of thought. Book 2 deals with the “logical” part of philosophy, Book 3 with the physical and ethical parts.
Here’s the opening paragraph in Outlines of Pyrrhonism:
Those who investigate any subject are likely either to make a discovery, or to deny the possibility of discovery and agree that nothing can be apprehended, or else to persist in their investigations. That, no doubt, is why of those who undertake philosophical investigations some say that they have discovered the truth, others deny the possibility of apprehending it, and others are still pursuing their investigations. Those who are properly called dogmatists - such as the Aristotelians and the Epicureans and the Stoics and others - think they have discovered the truth; Clitomachus and Carneades and other Academic philosophers have said that the truth cannot be apprehended; and the sceptics persist in their investigations.According to Sextus, the sceptics are students or researchers who ‘persist in their investigations’. The sceptic students or researchers must persist in their investigations because they have not discovered the object that they are searching for and they have not reached the conclusion that what they search for is beyond their powers of investigation. They have no opinion on the matter and they keep investigating. So the sceptics are the ones who have suspended judgement, they neither believe nor disbelieve, neither affirm or deny. They doubt everything and there is no end to their doubt. The Greek term “skeptikos” means “to inquire” or “to consider”.
Pyrrhonian skepticism can be seen as a reaction to the schools that Sextus calls dogmatic: ‘Aristotelians and the Epicureans and the Stoics’. He accuses them of making claims that cannot be verified through actual observations. During the time of Sextus, the word “dogmatic” did not have the pejorative tone that it has today—in that period a dogmatic was someone who was stickler for dogmas or doctrines.