Theaetetus: And do you not agree in that view, Socrates?
Socrates: If you do, my friend; but I want to know first, whether you admit the resolution of all things into their elements to be a rational explanation of them, and the consideration of them in syllables or larger combinations of them to be irrational—is this your view?
Theaetetus: Precisely.
Socrates: Well, and do you conceive that a man has knowledge of any element who at one time affirms and at another time denies that element of something, or thinks that the same thing is composed of different elements at different times?
Theaetetus: Assuredly not.
Socrates: And do you not remember that in your case and in that of others this often occurred in the process of learning to read?
Theaetetus: You mean that I mistook the letters and misspelt the syllables?
Socrates: Yes.
Theaetetus: To be sure; I perfectly remember, and I am very far from supposing that they who are in this condition have knowledge.
Socrates: When a person at the time of learning writes the name of Theaetetus, and thinks that he ought to write and does write th and e; but, again, meaning to write the name of Theododorus, thinks that he ought to write and does write t and e—can we suppose that he knows the first syllables of your two names?
Theaetetus: We have already admitted that such a one has not yet attained knowledge.
Socrates: And in like manner be may enumerate without knowing them the second and third and fourth syllables of your name?
Theaetetus: He may.
Socrates: And in that case, when he knows the order of the letters and can write them out correctly, he has right opinion?
Theaetetus: Clearly.
Socrates: But although we admit that he has right opinion, he will still be without knowledge?
Theaetetus: Yes.
Socrates: And yet he will have explanation, as well as right opinion, for he knew the order of the letters when he wrote; and this we admit to be explanation.
Theaetetus: True.
Socrates: Then, my friend, there is such a thing as right opinion united with definition or explanation, which does not as yet attain to the exactness of knowledge.
Theaetetus: It would seem so.
Socrates: And what we fancied to be a perfect definition of knowledge is a dream only. But perhaps we had better not say so as yet, for were there not three explanations of knowledge, one of which must, as we said, be adopted by him who maintains knowledge to be true opinion combined with rational explanation? And very likely there may be found someone who will not prefer this but the third.
Theaetetus: You are quite right; there is still one remaining. The first was the image or expression of the mind in speech; the second, which has just been mentioned, is a way of reaching the whole by an enumeration of the elements. But what is the third definition?
Socrates: There is, further, the popular notion of telling the mark or sign of difference which distinguishes the thing in question from all others.
Theaetetus: Can you give me any example of such a definition?