Although there were many differences among the Sophists in terms of their specific teachings, it is safe to say that there was a common philosophy which many Sophists shared and which permeated their teachings. The relativity of truth was the basis of Protagoras's rhetorical teaching. He trained his students to argue on both sides of a question because he believed that the whole truth could not be limited to just one side of a question. Therefore, he taught his students to praise and blame the same things and to strengthen the weaker argument so that it might appear the stronger. These techniques are based on the belief that truth is relative to the individual.
Truth is what it appears to be to the individual.
If an action is advantageous to the individual, then it is good. This idea was sometimes employed by the unscrupulous to justify morally questionable behavior, but Protagoras apparently was opposed to an indiscriminate use of this principle. His belief in the relativity of truth did not prevent him from believing that in making moral decisions one can still distinguish between an action which is morally better and one that is morally worse. The Sophists were also interested in the cultural development of man as a member of society. The Sophists saw man himself as a product of nature, but society and civilization as artificial human products.