soul spirit is arrayed on the side of the rational principle.

Most assuredly.

But a further question arises: Is passion different from reason also, or only a kind of reason; in which latter case, instead of three principles in the soul, there will only be two, the rational and the concupiscent; or rather, as the State was composed of three classes, traders, auxiliaries, counsellors, so may there not be in the individual soul a third element which is passion or spirit, and when not corrupted by bad education is the natural auxiliary of reason?

Yes, he said, there must be a third.

Yes, I replied, if passion, which has already been shown to be different from desire, turn out also to be different from reason.