SOC. Take an example: When at such a stage in his progress a person in writing Theaetetus thinks he ought to write,

 
and actually does write, TH and E, and again in trying to write Theodorus thinks he ought to write, and does write, T and E, shall we say that he knows the first syllable of your names? THEAET. No, we just now agreed that a person in such a condition has not yet gained knowledge. SOC. Then there is nothing to prevent the same person from being in that condition with respect to the second and third and fourth syllables? THEAET. No, nothing. SOC. Then, in that case, he has in mind the orderly description in terms of letters, and will write Theaetetus with right opinion, when he writes the letters in order? THEAET. Evidently. SOC. But he is still, as we say, without knowledge, though he has right opinion? THEAET. Yes. SOC. Yes, but with his opinion he has rational explanation; for he wrote with the method in terms of letters in his mind, and we agreed that that was rational explanation. THEAET. True. SOC. There is, then, my friend, a combination of right opinion with rational explanation, which cannot as yet properly be called knowledge? THEAET. There is not much doubt about it. SOC. So it seems that the perfectly true definition of knowledge, which we thought we had, was but a golden dream. Or shall we wait a bit before we condemn it? Perhaps the definition to be adopted is not this,
 
but the remaining one of the three possibilities one of which we said must be affirmed by anyone who asserts that knowledge is right opinion combined with rational explanation. THEAET. I am glad you called that to mind. For there is still one left. The first was a kind of vocal image of the thought, the second the orderly approach to the whole through the elements, which we have just been discussing, and what is the third? SOC. It is just the definition which most people would give, that knowledge is the ability to tell some characteristic by which the object in question differs from all others. THEAET. As an example of the method, what explanation can you give me, and of what thing? SOC. As an example, if you like, take the sun: I think it is enough for you to be told that it is the brightest of the heavenly bodies that revolve about the earth. THEAET. Certainly. SOC. Understand why I say this. It is because, as we were just saying, if you get hold of the distinguishing characteristic by which a given thing differs from the rest, you will, as some say, get hold of the definition or explanation of it; but so long as you cling to some common quality, your explanation will pertain to all those objects to which the common quality belongs. THEAET. I understand; and it seems to me that it is quite right to call that kind a rational explanation or definition. SOC. Then he who possesses right opinion about anything and adds thereto a comprehension of the difference which distinguishes it from other things will have acquired knowledge of that thing of which he previously had only opinion. THEAET. That is what we affirm. SOC. Theaetetus, now that I have come closer to our statement, I do not understand it at all. It is like coming close to a scene-painting. In which perspective is the main thing. While I stood off at a distance, I thought there was something in it. THEAET. What do you mean?