How did Protagoras connect moral rules with social convention?

Protagoras, a prominent Sophist, connected moral rules to social convention by arguing that concepts of good, bad, just, and unjust do not reflect objective facts of nature. Instead, he posited that these standards are human inventions determined by the consensus of a specific society.12

His famous dictum, "man is the measure of all things," implies that moral values are relative to the individual and the community. As Plato represented his views, whatever a city deems just and fine is considered valid for that city as long as it maintains that belief.341

While Protagoras viewed moral rules as conventional rather than divinely or metaphysically ordained, he maintained a moderate stance on their necessity. He argued that although the specific content of moral rules varies between cultures, some form of social regulation is essential to make communal life tolerable. By grounding ethics in human experience and social agreement, he moved away from the search for absolute, objective moral truths, focusing instead on the practical requirements of living within a society.1