Outline of Plato's Theaetetus


Protagoras proposed that Man is the Measure:

Knowledge is identical with perception.

Basic formula: (I know x) = (I perceive x)                
Example: (I know I’m cold) = (I perceive that I am cold)

Perception is relative to the perceiver. (I may feel cold when you feel hot)

Knowledge is subjective, but no appearance/reality gap.


Argument that they are equivalent:

  1. Perception is “inseparable” from the perceiver
    & only the perceiver can judge what is perceived.

  2. No room for error in judging own perception.

        Therefore I know by perceiving.

Additional consequence: Knowledge is always in flux (because perception is).


Objection based on memory:

  1. If one remembers what was known through perception, one knows whatever content it is that one remembers.

  2. But (from above) perception = knowledge.

So when the perception is finished, the knowledge is gone.

So in memory, one knows and does not know the same content.

But this is impossible.

So knowledge can’t equal perception.

Since the argument starts by assuming that Protagoras is correct, then derives a bad result, this is a reductio ad absurdum.


Objection from false opinion

  1. Everyone thinks some others wiser & some others less wise (e.g., we choose who should guide us when we face trouble)

  2. This behavior assumes a difference between true and false opinion. (People at least perceive others to be wiser or less wise.)

  3. But to equate knowledge with perception is to deny this distinction: if “man is the measure” is true, then common practice (dismissing some people as foolish) measures it as false. But if we say that the opinion of others is false, then we deny that man is the measure. (For the person who holds that “man is the measure” must grant that others have true opinion, including about this.)

To avoid this paradox, we should grant the people differ in their wisdom.


Argument that perception cannot be knowledge

  1. Each type of perception is distinct from the rest (the various senses perceive “unconnected” parts of the same object).

  2. But to know that sounds differ from colors cannot be an act of perception. (We know “both are two” and the two are “unlike” each other)

  3. “Likeness” and “unlikeness” are universals, “unlike objects of sense” to which they are applied.

  4. To apply them requires a ”power” that is not a perception. (If one used only one’s perception, one would “miss the truth” that they are unlike.)

Therefore we know something that is not known by sensing/perceiving.

Therefore knowledge is different from perception.