SOCRATES: Attend to what follows: must not the perfect arithmetician know all numbers, for he has the science of all numbers in his mind?
THEAETETUS: True.
SOCRATES: And he can reckon abstract numbers in his head, or things about him which are numerable?
THEAETETUS: Of course he can.
SOCRATES: And to reckon is simply to consider how much such and such a number amounts to?
THEAETETUS: Very true.
SOCRATES: And so he appears to be searching into something which he knows, as if he did not know it, for we have already admitted that he knows all numbers;—you have heard these perplexing questions raised?
THEAETETUS: I have.
SOCRATES: May we not pursue the image of the doves, and say that the chase after knowledge is of two kinds? one kind is prior to possession and for the sake of possession, and the other for the sake of taking and holding in the hands that which is possessed already. And thus, when a man has learned and known something long ago, he may resume and get hold of the knowledge which he has long possessed, but has not at hand in his mind.
THEAETETUS: True.