In the Hellenistic and Roman periods after Plato, there were two kinds of Skeptics, and they were something like rivals: the Academics and the Pyrrhonists (stemming from Pyrrho of Elis, c. 360 – 270 B.C.). Sextus Empiricus was the last great Pyrrhonist. The Academics, according to 
Sextus, maintained that “all things are inapprehensible,” whereas the Pyrrhonists suspend judgement on all issues.
All things are inapprehensible, said the Academics. If that means that nothing can be known, then it certainly sounds pretty totally sceptical.
Is it true that nothing can be known? This is one of those philosophical questions that must be handled with a certain amount of care, for if it is true that nothing can be known, or if people came to believe this, then we can expect to see some fairly spectacular changes in lifestyles.
Fortunately, there seems to be an obvious and conclusive objection to this notion that nothing can be known. First of all, the claim that nothing can be known appears to be a knowledge-claim. Thus the claim that nothing can be known seems to be self-cancelling.
Second, if it is true that nothing can be known, then any argument used to establish that fact cannot be known either. Apparently, the idea that nothing can be known is thus not only self-cancelling, but also cannot be established on any grounds that are themselves claimed to be known.
It is little wonder, then, that Sextus differentiated his own position from that of the Academics. Sextus, unlike the Academics, did not proclaim that nothing can be known. Instead, he said, in effect: “I suspend judgment in the matter. Also, I suspend judgement on all other issues that I have examined too.” His position was, in short, that he did not know whether or not knowledge is possible.
Sextus’s version of total scepticism does not seem nearly so easy to refute as that of the Academics. And even today Sextus’s version of total scepticism has its adherents. Sextus did not affirm the possibility of knowledge of any sort, so it’s fair to call him a skeptic. But by not making any judgements- that is, by not commiting himself to any claims whatsoever, including the claim that knowledge is impossible- he did not put himself in the self-defeating position of claiming to know that he could not know.
You’re not totally convinced by Sextus? Perhaps, then, you have noticed that Sextus certainly appears to make judgements despite the fact that he says that he does not make them. For example, doesn’t he commit himself to a claim when he says that Pyrrhonists suspend judgement in all matters?
Sextus’s Rationale
Now what was Sextus’s rationale for suspending judgement on every issue? In his Outlines of Pyrrhonism, Sextus set forth the infamous “Ten Tropes,” a collection of ten arguments by the ancient sceptics against the possibility of knowledge. The idea behind the Ten Tropes is this. Knowledge possible only if we have good grounds for believing the world is exactly as we think it is or perceive it to be. The Ten Tropes suggest that because we perceive things differently that there is little reason to believe are perceptions are accurate.
In Sextus’s account, the basic ten tropes or formula arguments show that the same thing appears differently (1) to different animals due to their different abilities, (2) to different individuals due to their idiosyncrasies, (3) to different senses (an object has a different smell, texture, taste etc., (4) to the same sense in different conditions (i.e. dark & light, new & old) (5) in different positions, places, or distances (6) in combination with different things(light, air, moisture, solidity, heat etc.), (7) in different quantities, (8) in different relations or relative to something else , (9) if common or if rare, and (10) to people with different customs or ways of life.
Sextus’s main reason for thinking that one must suspend judgement on every issue, however, was that “to every argument an equal argument is opposed.” If this is true, then wouldn’t it indeed be rational to suspend judgement on every issue? (Thus, the balance scale, which represents the equally compelling force of two contradictory views, is the symbol of scepticism- as well as the scales of justice.)
