Now since some things will commingle and others will not,
they are in much the same condition as the letters of the alphabet; for some of these do not fit each other, and others do.
Of course.
And the vowels, to a greater degree than the others, run through them all as a bond, so that without one of the vowels the other letters cannot be joined one to another.
Certainly.
Now does everybody know which letters can join with which others? Or does he who is to join them properly have need of art?
He has need of art.
What art?
The art of grammar.
And is not the same true in connection with high and
low sounds? Is not he who has the art to know the sounds which mingle and those which do not, musical, and he who does not know unmusical?
Yes.
And we shall find similar conditions, then, in all the other arts and processes which are devoid of art?
Of course.
Now since we have agreed that the classes or genera also commingle with one another, or do not commingle, in the same way, must not he possess some science and proceed by the processes of reason who is to show correctly which of the classes harmonize with which, and which reject one another,
and also if he is to show whether there are some elements extending through all and holding them together so that they can mingle, and again, when they separate, whether there are other universal causes of separation?
Certainly he needs science, and perhaps even the greatest of sciences.
Then, Theaetetus, what name shall we give to this science? Or, by Zeus, have we unwittingly stumbled upon the science that belongs to free men and perhaps found the philosopher while we were looking for the sophist?
What do you mean?
Shall we not say that the division of things by classes and the avoidance of the belief that the same class is another, or another the same, belongs to the science of dialectic?
Yes, we shall.
Then he who is able to do this has a clear perception of one form or idea extending entirely through many individuals each of which lies apart, and of many forms differing from one another but included in one greater form, and again of one form evolved by the union of many wholes,
and of many forms entirely apart and separate. This is the knowledge and ability to distinguish by classes how individual things can or cannot be associated with one another.
Certainly it is.
But you surely, I suppose, will not grant the art of dialectic to any but the man who pursues philosophy in purity and righteousness.