But on the other hand, a particular knowledge is of a particular subject.

Again, every science has a distinct character, which is defined by an object; medicine, for example, is the science of health, although not to be confounded with health.

Having cleared our ideas thus far, let us return to the original instance of thirst, which has a definite object--drink.

Now the thirsty soul may feel two distinct impulses; the animal one saying 'Drink;' the rational one, which says 'Do not drink.' The two impulses are contradictory; and therefore we may assume that they spring from distinct principles in the soul.

But is passion a third principle, or akin to desire?
There is a story of a certain Leontius which throws some light on this question.