The two great Sophists Protagoras and Gorgias shifted the focus of Pre-Socratic philosophy from older attempts to elucidate Absolute Being to a more practical, humanistic system of thought. Whereas previous philosophers, such as those from the Milesian school or even atomists like Democritus, sought to discover the Absolute, that is, some form of independent, unchanging Truth, Protagoras and Gorgias rejected the notion that the Absolute even exists. Instead, they claimed that Truth is relative to the person looking for it.
As Protagoras said, "Of all things human being is the measureâ€¦". In other words, people, interacting within a society, determine that society's values. Values are relative to the society which holds them. Furthermore, Protagoras held that it is impossible for an individual to truly know any absolute Truth. Thus our primary concern must be with the ways things appear to be to us and deriving, from those appearances, customary patterns of practical action. For this reason, nomos, custom, is far more important to the Sophists than nature or Absolute Truth. The way we form said customs is, according to Protagoras and especially Gorgias, through clever argument and rhetoric. In a political community, the ideas backed by the most convincing arguments influence our fellows' doxa, or opinions, and, as such, are the ones which are adopted as custom. Thus, skilled speech, rhetoric, and opinion determine what constitutes truth for us.
There is one main problem with this type of practical relativism. First of all, as other Pre-Socratics such as Parmenides theorized, there may simply be a radical divorce between doxa and Truth. Certainly we may use Sophistic theories to function better in the world of doxa, but this does not change the simple fact that there is an Absolute behind the world of everyday opinions and appearances. Perhaps true knowledge of such an Absolute would be inapplicable to the everyday world. In fact, in Parmenides' own theory, one must act as though doxa were the whole story of reality in order to lead a normal, day to day life! This doesn't mean that the existence of the Absolute should be denied or dismissed out of hand, or that knowledge of the Absolute is not valuable. It is the philosopher's job to seek out exactly that knowledge. By rejecting both the search for and the existence of the Absolute, the Sophists have, effectively, stopped practicing philosophy in favor of (admittedly very useful) practical statecraft.