_But in his conclusions as to the value of human nature_, Socrates set himself entirely against the outcome of the reflections of the Sophists, and indeed of his time. In the absorbing anthropological topics of his time, he laid the foundations of a constructive philosophy against the skeptical conclusions of the Sophists. In human matters he maintained that there is a validity to truth and a possibility of absolute knowledge. He admitted with the Sophists that there are obscurities in human thought, and that obviously the standard of truth does not belong to any one man. But while the Sophists emphasized these contradictions and reasoned therefore that no valid truth existed, Socrates cut his way through such contradictions and obscurities, emphasized the identity in men, and maintained that the truth is in all men together,――in humanity. It exists as an ideal to be striven for by men together. When Protagoras says that “man is the measure of all things,” he means by “man” the individual man; while Socrates, if he had used that expression, would have meant “humanity.” And Socrates means by his principle “virtue is knowledge” that the knowledge of that same humanity (_i. e._ insight, reason) is virtue; while Protagoras, agreeing as he did formally with the maxim that “virtue is knowledge,” would always define “knowledge” as the individual feelings. “The individual man is the measure of all things,” Protagoras would say; “Humanity is the measure of all things,” Socrates would reply. “Virtue is knowledge gained by the feelings,” Protagoras would say; “Virtue is knowledge gained by the reason,” Socrates would reply. Beneath the changing capricious individual, beneath the variety of men, Socrates believed that there was a common humanity, one unchanging man, who contained the ultimate truth. There are many opinions, ideas, and feelings, but only one knowledge. This knowledge is rational; and human nature is a unity in the possession of this knowledge.