"The Greek word skepsis means investigation. By calling themselves skeptics, the ancient skeptics thus describe themselves as investigators. They also call themselves ‘those who suspend’ (ephektikoi), thereby signaling that their investigations lead them to suspension of judgment. They do not put forward theories, and they do not deny that knowledge can be found. At its core, ancient skepticism is a way of life devoted to inquiry. Also, it is as much concerned with belief as with knowledge. As long as knowledge has not been attained, the skeptics aim not to affirm anything. This gives rise to their most controversial ambition: a life without belief."
"Arcesilaus (316/5-241/0 BCE) was a member and later leader (‘scholarch’) of Plato's Academy. He initiated the skeptical phase of the Platonic school (‘Academic skepticism’) and was an influential critic of Stoic epistemology.
The ancient evidence about Arcesilaus' philosophy is difficult to evaluate and, in some respects, inconsistent. As a result, scholars interpret his skepticism in several ways. Some see his philosophical activity as entirely negative or destructive of all philosophical views. Others regard him as endorsing the position that nothing can be known on the basis of philosophical arguments he is committed to. This would make his skepticism similar to Descartes' at the start of the Meditations, where he doubts everything he previously thought he knew. Others take him to have held no positive views on any philosophical topic, including the possibility of knowledge. On this view, his skepticism is very similar to the Pyrrhonian skepticism advocated by Sextus Empiricus: the skeptic is someone who refuses to accept any philosophical theory or proposition as rationally warranted, and insists that further examination is always required."
"Carneades (214–129/8 B.C.E.) was a member and eventually scholarch or head of the Academy, the philosophical school founded by Plato, for part of its skeptical phase. He is credited by ancient tradition with founding the New or Third Academy and defended a form of probabilism in epistemology."
"Carneades was perhaps the most prominent head of the skeptical Academy in ancient Greece. Following the example of Arcesilaus, who turned the Academy in a skeptical direction, Carneades developed an array of arguments against the dogmatic positions upheld by other philosophers, particularly the Stoics. He went beyond Arcesilaus in several respects, however. Instead of simply arguing against the positive positions of other philosophers, Carneades also set forth arguments of his own in favor of views that sometimes had never been defended before–not in order to establish their truth, but simply to counterbalance the arguments of the dogmatists and show that none of their conclusions can be conclusively established. In doing so, Carneades made important contributions to several philosophical debates. Carneades also set forth a more detailed skeptical criterion of what to believe, to pithanon which means either the 'plausible' or the 'probable.'"
"Pyrrho was the starting-point for a philosophical movement known as Pyrrhonism that flourished beginning several centuries after his own time. This later Pyrrhonism was one of the two major traditions of sceptical thought in the Greco-Roman world (the other being located in Plato’s Academy during much of the Hellenistic period). Perhaps the central question about Pyrrho is whether or to what extent he himself was a sceptic in the later Pyrrhonist mold. The later Pyrrhonists claimed inspiration from him; and, as we shall see, there is undeniably some basis for this. But it does not follow that Pyrrho’s philosophy was identical to that of this later movement, or even that the later Pyrrhonists thought that it was identical; the claims of indebtedness that are expressed by or attributed to members of the later Pyrrhonist tradition are broad and general in character (and in Sextus Empiricus’ case notably cautious—see Outlines of Pyrrhonism 1.7), and do not in themselves point to any particular reconstruction of Pyrrho’s thought."