Socrates: As, for example, in the case of the sun, I think that you would be contented with the statement that the sun is the brightest of the heavenly bodies which revolve about the earth.
Theaetetus: Certainly.
Socrates: Understand why:—the reason is, as I was just now saying, that if you get at the difference and distinguishing characteristic of each thing, then, as many persons affirm, you will get at the definition or explanation of it; but while you lay hold only of the common and not of the characteristic notion, you will only have the definition of those things to which this common quality belongs.
Theaetetus: I understand you, and your account of definition is in my judgment correct.
Socrates: But he, who having right opinion about anything, can find out the difference which distinguishes it from other things will know that of which before he had only an opinion.
Theaetetus: Yes; that is what we are maintaining.
Socrates: Nevertheless, Theaetetus, on a nearer view, I find myself quite disappointed; the picture, which at a distance was not so bad, has now become altogether unintelligible.
Theaetetus: What do you mean?
Socrates: I will endeavour to explain: I will suppose myself to have true opinion of you, and if to this I add your definition, then I have knowledge, but if not, opinion only.
Theaetetus: Yes.
Socrates: The definition was assumed to be the interpretation of your difference.
Theaetetus: True.
Socrates: But when I had only opinion, I had no conception of your distinguishing characteristics.
Theaetetus: I suppose not.
Socrates: Then I must have conceived of some general or common nature which no more belonged to you than to another.
Theaetetus: True.
Socrates: Tell me, now—How in that case could I have formed a judgment of you any more than of anyone else? Suppose that I imagine Theaetetus to be a man who has nose, eyes, and mouth, and every other member complete; how would that enable me to distinguish Theaetetus from Theodorus, or from some outer barbarian?
Theaetetus: How could it?
Socrates: Or if I had further conceived of you, not only as having nose and eyes, but as having a snub nose and prominent eyes, should I have any more notion of you than of myself and others who resemble me?
Theaetetus: Certainly not.
Socrates: Surely I can have no conception of Theaetetus until your snub-nosedness has left an impression on my mind different from the snub-nosedness of all others whom I have ever seen, and until your other peculiarities have a like distinctness; and so when I meet you tomorrow the right opinion will be recalled?
Theaetetus: Most true.
Socrates: Then right opinion implies the perception of differences?
Theaetetus: Clearly.
Socrates: What, then, shall we say of adding reason or explanation to right opinion? If the meaning is, that we should form an opinion of the way in which something differs from another thing, the proposal is ridiculous.