Suppose we call it the contentious or ambitious — would the term be suitable?

Extremely suitable.

On the other hand, every one sees that the principle of knowledge is wholly directed to the truth, and cares less than either of the others for gain or fame.

Far less.

‘Lover of wisdom,’ ‘lover of knowledge,’ are titles which we may fitly apply to that part of the soul?

Certainly.

One principle prevails in the souls of one class of men, another in others, as may happen?

Yes.

Then we may begin by assuming that there are three classes of men — lovers of