Why did Protagoras think societies need moral rules even when their content differs?

Protagoras, a prominent Sophist, argued that while the specific content of moral rules is a matter of social convention rather than objective fact, societies must still maintain some form of moral framework to ensure that life remains tolerable.1

Although he famously asserted that "man is the measure of all things," suggesting that what is considered "just and fine" is relative to each city, he did not advocate for a total absence of rules. Instead, he maintained that for a community to function and for human life to be sustainable, there must be established norms of conduct. By grounding his view in the necessity of social order rather than in divine or metaphysical mandates, Protagoras emphasized that moral rules are essential practical tools for human coexistence, even if their specific details vary between different cultures.21