Plato's The Theaetetus





Man is the Measure of All Things,


of the Things That Are, That They Are, and


of the Things That Are Not, That They are Not.





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Each and Every Event is For Me as it Appears for Me


and For You as it Appears for You

























































Philosophy 290-29B


April 1, 2004








In this paper I will examine the discussions between Socrates and Theaetetus concerning Protagoras' proposal that "man is the measure of all things, of the things that are, that they are, and of the things that are not, that they are not" and it's tie to the proposition that "every event is for me as it appears to me and for you as it appears for you." To this end, I will summarize the various arguments that Socrates and Theaetetus pose against the Protagorian proposal and their eventual rejection of it. I will offer several responses one might make to defend Protagoras' position and finally support my position that perception is not knowledge.


Plato's Theaetetus is concerned with the question "What is knowledge?" Though the dialogue never fully answers the question, it does deal with several interesting proposals along the way. One such proposal offered by Theaetetus is "It seems to me that one who knows something is perceiving the thing he knows, and, so far as I can see at present, knowledge is nothing but perception." Socrates points out that this suggestion is a re-wording of Protagoras' proposition that "man is the measure of all things". If perception equals knowledge, what is perceived does not just seem to me, but is for me, therefore; perception is reality, not just opinion. Socrates summarizes this notion, "that any given thing is to me as it appears to me, and is to you such as it appears to you." He supports this with an example when one person feels the wind is chilly and the other does not, or one feels the wind slightly chilly and the other feels it quite cold. Presumably Protagoras did not mean that the wind was both chilly and not chilly. His point here is more of an epistemological view that corresponds to Heraclitus' metaphysical view of reality being in flux. In this sense, the metaphysical model of Heraclitus might make the relativism of Protagoras plausible. Socrates suggests as much when he says, "We were wrong to describe things in that way, because nothing ever is, but is continually being generated. The whole succession of past sages (with the exception of Parmenides) can be seen to agree on this point - I'm thinking of Protagoras, Heraclitus and Empedocles - and so can Epicharmus and Homer...". Protagoras' philosophy, being a relativist, is that everything exists in our minds which is the opposite of Socrates' objectivist view that everything exists outside our minds, independent of our reasoning. Therefore Socrates raises that this violates the law of non-contradiction, something cannot be two different ways at once. The premise that outside forces may effect our perception is brought into the argument. He points out that perceptions vary in one person when that person is healthy vs. being ill. He gets Theaetetus to support this by admitting that when someone is mad or dreaming, one can perceive what is not the case. But if this is so, then this seems to conflict with the Protagorian dictum that whatever one perceives is knowledge. Socrates furthers this view by arguing that the Socrates who is healthy and the Socrates who is ill are two different subjects. Thus, the wine tasting sweet to the healthy Socrates and the wine tasting bitter to the ill Socrates can both be true. Given this, "since what acts upon me is for me and for no one else, I, and no none else, am actually perceiving it...Then my perception is true for me, for its object at any moment is my reality, and I am, as Protagoras says, a judge of what is for me, that it is, and of what is not, that it is not...." Socrates' most damaging argument to this Protagorian dictum comes when he states that, since whatever anyone thinks is true, then for those who think that Protagoras' own view is false, it follows that it is true that Protagoras' proposal is false. Protagoras, to maintain