Skepticism --The problem of the criterion, munchausen problem / agrippa's trilemma

http://en.wikipedia.org/wiki/Problem_of_the_criterion
American philosopher Roderick M. Chisholm in his Theory of Knowledge details the problem of the criterion with two sets of questions:

1. What do we know? or What is the extent of our knowledge?

2. How do we know? or What is the criterion of knowing? 

http://en.wikipedia.org/wiki/Regress_argument 

http://en.wikipedia.org/wiki/Agrippa%27s_Trilemma 

The following tropes for Greek skepticism are given by Sextus Empiricus, in his Outlines of Pyrrhonism. According to Sextus, they are attributed only "to the more recent skeptics" and it is byDiogenes Laertius that we attribute them to Agrippa.^[1] The tropes are:

1. Dissent - The uncertainty of the rules of common life, and of the opinions of philosophers.
2. Progress ad infinitum - All proof requires some further proof, and so on to infinity.
3. Relation - All things are changed as their relations become changed, or, as we look upon them from different points of view.
4. Assumption - The truth asserted is merely a hypothesis.
5. Circularity - The truth asserted involves a vicious circle (see regress argument, known in scholasticism as diallelus)

[165] According to the mode deriving from dispute, we find that undecidable dissension about the matter proposed has come about both in ordinary life and among philosophers. Because of this we are not able to choose or to rule out anything, and we end up with suspension of judgment. [166] In the mode deriving from infinite regress, we say that what is brought forward as a source of conviction for the matter proposed itself needs another such source, which itself needs another, and so ad infinitum, so that we have no point from which to begin to establish anything, and suspension of judgment follows. [167] In the mode deriving from relativity, as we said above, the existing object appears to be such-and-such relative to the subject judging and to the things observed together with it, but we suspend judgment on what it is like in its nature. [168] We have the mode from hypothesis when the Dogmatists, being thrown back ad infinitum, begin from something which they do not establish but claim to assume simply and without proof in virtue of a concession. [169] The reciprocal mode occurs when what ought to be confirmatory of the object under investigation needs to be made convincing by the object under investigation; then, being unable to take either in order to establish the other, we suspend judgment about both.^[2]