How did Sophists argue that moral rules were based on social convention?

The Sophists argued that moral rules were based on social convention—a concept known in Greek as nomos—rather than objective facts of nature (physis). This perspective emerged from their observations that different societies maintained varying rules of conduct regarding sacred matters, such as marriage, burial, and relations between the sexes.123

Key aspects of this argument included:

• Human Invention: Sophists viewed laws and moral codes as human inventions established by consensus to restrict natural freedoms for the sake of expediency and self-interest.1
• Relativism: Protagoras, famous for the dictum "Man is the measure of all things," suggested that what is considered "just and fine" is relative to each city and its own beliefs.42
• Skepticism of Objective Morality: Some Sophists, such as Thrasymachus, took a more radical approach, arguing that justice is merely the interest of the stronger party, as laws are created by those in power to serve their own ends.23

By rejecting the idea that moral norms were grounded in a divine or metaphysical realm, the Sophists shifted the focus of ethics toward human society and the practical needs of the individual.2