Agrippa was a Sceptic philosopher who probably lived towards the end of the 1st century A.D. He is regarded as the author of the five tropes 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:
Explanation of the tropes
 That which leaves the dissension is that by which we discover as in connection with the examined thing it was, as well in the everyday life as among the philosophers, a dissension indécidable which prevents us choosing something or from rejecting it, us finally leading to the suspension of the jugement.  That which is based on the regression ad infinitum is that in which we say that what is provided in order to carry the conviction on the thing proposed with the examination has need and that ad infinitum, so that, not having anything from what we will be able to start to establish something, the suspension of the approval ensuit.  the mode according to the relative one, as we mentioned above, is that in which the object appears such or such relative with what judges and with what is observed jointly, and on what it is according to nature we suspend our assentiment.[168 ] We have the mode which starts from an assumption when the dogmatic ones being returned ad infinitum, they start from something which they do not establish but judge good to take simply and without demonstration what is used to ensure the thing to which relates research needs this thing to carry the conviction; then not being able to take one to establish the other, we suspend our approval on both.
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.
This page is an edited Google-translation of the French wikipedia page.
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