Final answer:
Protagoras was a seminal figure in the realm of philosophical relativism, believing subjective experience dictated truth. Plato countered this with the notion of objective knowledge as justified true belief. Both philosophers contributed significantly to the discourse on truth in ancient Greek philosophy.
Step-by-step explanation:
Protagoras, a preeminent Sophist, posited that "Man is the measure of all things," indicating a form of philosophical relativism, where truth is subjective and varies from individual to individual. Unlike Socrates, who emphasized critical reasoning for uncovering objective truths, Protagoras believed in individual perception as the ultimate determiner of one's reality. Plato, in his depiction of Protagoras' philosophy in the dialogue Theaetetus, contrasts this relativism with his own belief in the existence of objective truths, arguing that knowledge must be justified true belief.
Plato's own perspective stemmed from his analysis of the metaphysical debates between the monists and pluralists, and his recognition, along with Socrates, of the inadequacy of Greek mythology to account for the complexities of moral and social order. The progression towards a more logical and reason-based understanding of realities led Plato to define knowledge as justified true belief, integrating the ideas of reality's constant change by Heraclitus and the illusion of change by Parmenides into his own metaphysical framework. This philosophical journey is what underpins the foundation of Western philosophical thought.