Protagoras is a sophist mentioned by Plato in his dialogue Protagoras. His theory is considered the first relativist theory. It stated that everything that seems true to me, must be true. This would imply that there is no absolute truth outside of humans, a truth that "can be known." We can only know truth from what we can perceive. Therefore, truth is relative to our individual perception. This view is summarized in his famous statement:
"Man is the measure of all things: of the things that are, that they are, of the things that are not, that they are not."
On the other hand, Socrates was a proponent of a dialectic method (now called the Socratic method). It consists of trying to undermine a hypothesis by pointing out mistakes or contradictions in it. The idea is that hypothesis elimination is a way of perfecting an argument. While Socrates believed that knowledge was possible, and truth existed, the first part of getting close to it was to acknowledge one's own ignorance. It is also a method that places a lot of emphasis on coherence as a way to reach truth.
