Why did Protagoras believe societies still need moral rules even when rules vary?

Protagoras, a prominent Sophist, argued that moral rules are matters of social convention rather than objective facts of nature. While he famously asserted that "man is the measure of all things," implying that what is considered "just" or "fine" varies between different cities and cultures, he did not advocate for a lawless society.12

Instead, Protagoras maintained that moral rules are essential for human life to be tolerable. He believed that even though the specific content of these rules is relative and human-made, a society must establish some form of order to function effectively. By grounding ethics in human consensus rather than divine or metaphysical authority, he emphasized that the necessity of rules arises from the practical requirements of communal living. His perspective highlights the ethical principle that social stability and the preservation of a functional community are fundamental goods that require the collective adoption of shared standards, regardless of their origin.23