What did Protagoras mean by saying that humans are the measure of all things?

The dictum "man is the measure of all things," attributed to the Sophist Protagoras, serves as a foundational expression of relativism regarding human perception and judgment.123

Rather than asserting that objective reality does not exist, Protagoras argued that for human beings, the world is defined by how it appears to the individual. He suggested that things are for each person exactly as they are perceived by that person. To illustrate this, he noted that it is futile to tell someone who is shivering with cold that it is "really" warm; for that individual, the cold is a reality because it is what they are experiencing.23

By focusing on the observer, Protagoras shifted the philosophical emphasis away from the abstract, often contradictory speculations about the physical world favored by his predecessors. His stance implies that all perceptions—and potentially all judgments—are relative to the individual percipient, making the human experience the standard by which reality is understood.4231