G. Reale, A History of Ancient Philosophy: The Schools of the Imperial Age, trans. John R. Catan (Albany: State University of New York Press, 1990) 141.
Reale quotes Sextus, a Neo-Skeptic who lived in the second century C.E.:
So also when we say that no proof exists we imply in our statement the exception of the argument which proves that proof does not exist; for this alone is proof. And even if it does banish itself, the existence of proof is not thereby confirmed. For there are many things which produce the same effect on themselves that they produce on other things. Just as, for example, fire after consuming the fuel destroys also itself, and like purgatives after driving the fluids out of the bodies expel themselves as well, so too the argument against proof, after abolishing every proof, can cancel itself also!
In most arguments about relativism and post-modernism that I've encountered, the "pro" side claims that there are no absolutes and holds that metanarratives are subjective stories rather than objective fact, and the "anti" side then retorts that the relativist or post-modernist is positing his own "absolute truth" and metanarrative. I once read a Christian book that casually dismissed relativism in this manner: "You say, O relativist, that there is no absolute truth. Well, you obviously believe that relativism is absolute truth, since you're saying that there are no absolutes! So your system of thought contradicts itself, and is therefore invalid." The skeptic Sextus had to deal with this argument in the second century C.E.!
Granted, relativism is self-contradictory in that it makes its own set of truth-claims, but does that mean that it's totally wrong? There are many things that we say or think that are rooted in our perceptions, culture, and biases, all of which color our view of humanity. What was "absolute truth" yesterday is not considered "absolute truth" today, so why should we be so dogmatic?
At the same time, it would be nice to see relativists showing humility themselves rather than replacing one dogmatism with another: their own.