Two Ways to Take Protagoras
Protagoras is famous for claiming that man is the measure of all things: of those that are, that they are; and of those that are not, that they are not. I’ll call this the MM thesis. It remains an influential worldview – particularly for those who say things like: “We each have our own truths” and “There is nothing either good or bad, but thinking makes it so.” (Hamlet, Act 2, Scene 2)
Now, there are at least two ways to interpret the MM thesis. I’ll call them the Radical Relativism Interpretation (RRI) and the Practical Interpretation (PI). The former is deeply problematic but the latter is plausible.
On the RRI, Protagoras means that truth is relative to each human being’s beliefs and/or desires. This is a radical version of cognitive relativism which entails a questionable moral subjectivism. There are some serious objections to the RRI. First, it is self-refuting. If truth is relative to each human being’s beliefs, then every belief is true. Hence, the belief that MM is false is also true, which refutes MM. Second, experience indicates that some beliefs are false — such as when Jones believes on faulty memory that he left his keys in the kitchen only to discover that he left them in the living room — and thus the MM fails the practical test of experience. Third, if MM holds that truth is relative to each human being’s desires, then the MM entails that wishful thinking makes propositions true, which seems absurd. Can your wish that your bank account contains a billion dollars make the proposition true? Fourth, the MM seems to reduce to a form of solipsism which isolates the individual from the rest of the world. Fifth, which is a related point: since the MM makes truth radically subjective, the result is that it’s practically impossible for human beings to communicate with one another. I call this the Babel objection – for reasons you likely can understand if you’re familiar with the story in Genesis. Sixth, MM entails a moral subjectivism that is morally unacceptable. If truth is relative to each human being’s beliefs, then every belief is true, including every moral belief. But it is obvious that some moral beliefs are false, such as the belief of the corporate fraudster that corporate fraud is morally permissible. Seventh, if every belief is true, then there are no experts, no need for teachers, and no need for learning or education or advice from relevant authorties. But this is absurd. Thus, etc. (Socrates raises this kind of argument against Protagoras in Plato’s dialogues such as Thaeatetus.)
The PI seems to avoid these problems. According to the PI, we cannot know ultimate reality – at least not with epistemic certainty. In other words, we can’t know things in themselves, as Kant might put it. But we are rational agents. In the realm of phenomena, which is where we live (at least in this pre-mortem life), we must make reasonable choices and take fitting actions, which requires us to interpret our experiences, i.e., to measure them according to standards of rationality. In this sense, man is the measure of all his experiences in this phenomenal realm; it is up to each of us to determine responsibly which experiences are veridical and which are not.