SOCRATES: And that both are two and each of them one?
THEAETETUS: Yes.
SOCRATES: You can further observe whether they are like or unlike one another?
THEAETETUS: I dare say.
SOCRATES: But through what do you perceive all this about them? for neither through hearing nor yet through seeing can you apprehend that which they have in common. Let me give you an illustration of the point at issue:—If there were any meaning in asking whether sounds and colours are saline or not, you would be able to tell me what faculty would consider the question. It would not be sight or hearing, but some other.
THEAETETUS: Certainly; the faculty of taste.
SOCRATES: Very good; and now tell me what is the power which discerns, not only in sensible objects, but in all things, universal notions, such as those which are called being and not-being, and those others about which we were just asking—what organs will you assign for the perception of these notions?
THEAETETUS: You are thinking of being and not being, likeness and unlikeness, sameness and difference, and also of unity and other numbers which are applied to objects of sense; and you mean to ask, through what bodily organ the soul perceives odd and even numbers and other arithmetical conceptions.
SOCRATES: You follow me excellently, Theaetetus; that is precisely what I am asking.
THEAETETUS: Indeed, Socrates, I cannot answer; my only notion is, that these, unlike objects of sense, have no separate organ, but that the mind, by a power of her own, contemplates the universals in all things.