STRANGER: This communion of some with some may be illustrated by the case of letters; for some letters do not fit each other, while others do.
THEAETETUS: Of course.
STRANGER: And the vowels, especially, are a sort of bond which pervades all the other letters, so that without a vowel one consonant cannot be joined to another.
THEAETETUS: True.
STRANGER: But does every one know what letters will unite with what? Or is art required in order to do so?
THEAETETUS: Art is required.
STRANGER: What art?
THEAETETUS: The art of grammar.
STRANGER: And is not this also true of sounds high and low?—Is not he who has the art to know what sounds mingle, a musician, and he who is ignorant, not a musician?
THEAETETUS: Yes.