Agrippa the Skeptic
Agrippa (Greek: Ἀγρίππας) was a Skeptic philosopher who probably lived towards the end of the 1st century AD. He is regarded as the author of "five grounds of doubt" or tropes (Greek: τρόποι), which are purported to establish the impossibility of certain knowledge.
The Five Tropes
These tropes 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 by Diogenes Laertius that we attribute them to Agrippa. The tropes are:
- Dissent – The uncertainty of the rules of common life, and of the opinions of philosophers.
- Progress ad infinitum – All proof requires some further proof, and so on to infinity.
- Relation – All things are changed as their relations become changed, or, as we look upon them from different points of view.
- Assumption – The truth asserted is merely a hypothesis.
- Circularity – The truth asserted involves a vicious circle.
 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 judgement.  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 judgement follows.  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 judgement on what it is like in its nature.  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.  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 judgement about both.
With reference to these five tropes, that the first and third are a short summary of the ten original grounds of doubt which were the basis of the earlier skepticism. The three additional ones show a progress in the skeptical system, and a transition from the common objections derived from the fallibility of sense and opinion, to more abstract and metaphysical grounds of doubt.
According to Victor Brochard “the five tropes can be regarded as the most radical and most precise formulation of skepticism that has ever been given. In a sense, they are still irresistible today.” 
- Münchhausen trilemma
- Philosophical skepticism
- Sextus Empiricus
- Theory of justification
- Jowett, Benjamin (1867), "Agrippa (1)", in Smith, William, Dictionary of Greek and Roman Biography and Mythology 1, Boston: Little, Brown and Company, p. 77
- Diogenes Laërtius, ix.
- Sextus Empiricus, Pyrrhōneioi hypotypōseis i., from Annas, J., Outlines of Scepticism Cambridge University Press. (2000).
- Brochard, V., The Greek Skeptics.
|Wikisource has the text of the 1911 Encyclopædia Britannica article Agrippa.|
- Sextus Empiricus, Outlines of Pyrrhonism
- Diogenes Laertius, Lives of the Philosophers.
- Victor Brochard, The Greek Skeptics
- L. E. Goodman, "Skepticism", Review of Metaphysics 36: 819-848, 1983.
- Jonathan Barnes, The Toils of Scepticism, Cambridge 1990.