At this point in the dialogue, we have Theaetetus’ definition of knowledge, that it is sense perception, buttressed by the alleged insight of Protagoras that man is the measure of all things, and by the alleged insight of Heraclitus that everything is in flux. Now Socrates raises some preliminary objections that call into question the definition under consideration.
If we accept the fact that man is the measure of all things, Socrates asks, why not say that a pig is the measure of all things? Why not say a baboon is the measure of all things? Furthermore, there seems to be a great difference between having sense perception of something and having knowledge of it. For instance, to hear a foreign language is not to understand it. Again, one way of knowing something is through memory of a truth or a fact. But remembering is not perceiving through the senses; so perception cannot be the whole of knowledge. Socrates brings up these objections, but does not discuss them at length. They do seem to bring up an important issue: knowledge seems to be more than mere sense perception. Animals have sense perceptions, but, arguably, lack knowledge. Knowledge seems to require understanding and comprehension, not merely awareness of one’s environment.
Socrates pauses the objections to give Protagoras a chance to speak for himself. He imagines the sophist complaining about how Socrates is trying to refute his view to a mere boy. “When you try to examine one of my views through questioning, if the answerer gives the answer I would have given and is defeated, then I am refuted. But if he gives a different answer, then he alone is refuted.” Protagoras claims to have a nuanced view that will distinguish between someone who is wise and someone who is not. “So act as follows: do not take advantage of a thinker in your questioning. For it is pure hypocrisy to claim to care about honesty while you are misrepresenting the arguments of another.”[34] This is the first appearance in the history of philosophy of what is now call the principle of charity: the point that you should treat your opponents generously and attribute to them the best, most intelligent and coherent, not the worst, version of an argument or position.
Protagoras gives a preliminary defense of his Man the Measure doctrine. “I am far from denying the existence of wisdom and the wise person. This is the person I claim to be wise, whoever of us transforms the things that appear and are bad to make them appear and be good.” He gives as an example, a doctor who helps a sick person for whom good food tastes bitter, perceive good food to taste pleasant. One state (health) is better than the other (sickness), and so the doctor improves the patient’s perceptions.[35] “Of the appearances that some, through ignorance, call true, I say some are better than others, but not at all truer,” he concludes.[36]
This defense, however, raises problems of its own, even if Socrates does not immediately challenge it. How can it be that man is the measure of what is true, but not of what is good or better? And, if we take Protagoras’ example literally, it would appear a doctor could tell patients that they are fine, thereby improving their attitude, without actually healing them of their sicknesses. Doctors in fact make a distinction between your thinking you are sick (as a hypochondriac might) and your really being sick, and treat the disease, not merely the appearance of disease. Protagoras’ account, it seems, would turn all medical practice into psychological therapy.
Socrates responds to Protagoras’ doctrine first by making a general observation. According to Protagoras, a statement is not simply true, but true for the person uttering it. Why not apply Protagoras’ Man the Measure doctrine to the doctrine itself? According to Protagoras, man is the measure of all things: the sentence is true for Protagoras. But most people would not accept his doctrine: the sentence is false for them. If you took a Gallup poll, you would find that ‘Man is the measure of all things’ is true for a few people, and false for millions of others. Furthermore, Protagoras’ doctrine commits him to accepting everyone’s opinions as being valid; so he must accept that fact that his doctrine is false—for most people.[37] Now, strictly speaking, Protagoras’ view does not commit him to accepting the majority vote on any belief. But it does obligate him to say that a general thesis like Man the Measure is itself true only for those who accept it, and hence not universally valid. On his view, there can be no universal truths and hence no philosophical principles, not even his own. So his doctrine proves to be self-defeating.
Socrates now offers another objection. Consider the question of what is good for a sick person, with a view to the future? Will anyone’s opinion be valid, or will the doctor be in the best position to heal the patient? What about the value of a future meal? Will anyone’s opinion be valid, or should we turn the ingredients over to a skilled cook? And what will be the case for a community, with a view to what will be good for its future? Will anyone’s opinion be valid, or should we consult someone with experience in government? (This last case may seem problematic, given the complexities of a large society and the indeterminacy of future events. But we can distinguish between successful states and failed states in history, and do our best to maximize chances for success.) In every case, skill and expertise seem to have the advantage over mere subjective opinion.[38] It appears that man, in these sense of any random person, is not the measure of all things. Perhaps “man,” in the sense of the collective wisdom of the human race, including science, technology, and social science, does offer the best hope going forward—but that is not what Plato’s Protagoras has been arguing.
All of this comes down to a simple point. If ‘man is the measure of all things’ is true, then it must be true for someone. But if it is true for someone, it may be false for someone else. So if it is true, it is not universally true. In fact, if Man the Measure is a true thesis, there are no universal truths; indeed, even the claim that there are no universal truths is not universally true. The claim of universal relativism is, then, self-defeating.
[34] Plato Theaetetus 166a, d-e.
[35] Plato Theaetetus 166d-167a.
[36] Plato Theaetetus 167b.
[37] Plato Theaetetus 170b-171c.
[38] Plato Theaetetus 177c-179b.