An ongoing debate is centered on the difference between the sophists, who charged for their services, and Socrates, who did not. Instead of giving instruction Socrates professed a self-effacing and questioning posture, exemplified by what is known as the Socratic method (although Diogenes Laërtius wrote that Protagoras—a sophist—invented this method). Socrates' attitude towards the sophists was not entirely oppositional. In one dialogue Socrates even stated that the sophists were better educators than he was, which he validated by sending one of his students to study under a sophist. W. K. C. Guthrie classified Socrates as a sophist in his "History of Greek Philosophy".