Full Idea
Protagoras declares that it is possible to argue either side of any question with equal force, even the question whether or not one can equally argue either side of any question!
Gist of Idea
Protagoras says arguments on both sides are always equal
Source
report of Jaegwon Kim (Mind in a Physical World [1998]) by Seneca the Younger - Letters from a Stoic 088
Book Reference
Seneca: 'Letters from a Stoic (Selections)', ed/tr. Campbell,Robin [Penguin 1969], p.160
A Reaction
This is perhaps the most famous sceptical argument in the ancient world (though, note, Protagoras is most famous for his relativism rather than his scepticism). It is, of course, wrong. The arguments are sometimes equal, but often they are not.