STRANGER: For, surely, either all things have communion with all; or nothing with any other thing; or some things communicate with some things and others not. THEAETETUS: Certainly. STRANGER: And two out of these three suppositions have been found to be impossible. THEAETETUS: Yes. STRANGER: Every one then, who desires to answer truly, will adopt the third and remaining hypothesis of the communion of some with some. THEAETETUS: Quite true. 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. STRANGER: And we shall find this to be generally true of art or the absence of art. THEAETETUS: Of course. STRANGER: And as classes are admitted by us in like manner to be some of them capable and others incapable of intermixture, must not he who would rightly show what kinds will unite and what will not, proceed by the help of science in the path of argument? And will he not ask if the connecting links are universal, and so capable of intermixture with all things; and again, in divisions, whether there are not other universal classes, which make them possible? THEAETETUS: To be sure he will require science, and, if I am not mistaken, the very greatest of all sciences. STRANGER: How are we to call it? By Zeus, have we not lighted unwittingly upon our free and noble science, and in looking for the Sophist have we not entertained the philosopher unawares? THEAETETUS: What do you mean? STRANGER: Should we not say that the division according to classes, which neither makes the same other, nor makes other the same, is the business of the dialectical science? THEAETETUS: That is what we should say. STRANGER: Then, surely, he who can divide rightly is able to see clearly one form pervading a scattered multitude, and many different forms contained under one higher form; and again, one form knit together into a single whole and pervading many such wholes, and many forms, existing only in separation and isolation. This is the knowledge of classes which determines where they can have communion with one another and where not. THEAETETUS: Quite true. STRANGER: And the art of dialectic would be attributed by you only to the philosopher pure and true? THEAETETUS: Who but he can be worthy?