Aristotle, Nicomachean Ethics, I.13, 1102b13-1103a4
There seems to be also another irrational element in the soul - one which in a sense, however, shares in a rational principle. For we praise the reason of the continent man and of the incontinent, and the part of their soul that has reason, since it urges them aright and towards the best objects; but there is found in them also another natural element beside reason, which fights against and resists it. For exactly as paralyzed limbs when we intend to move them to the right turn on the contrary to the left, so is it with the soul; the impulses of incontinent people move in contrary directions. But while in the body we see that which moves astray, in the soul we do not. No doubt, however, we must nonetheless suppose that in the soul too there is something beside reason, resisting and opposing it. In what sense it is distinct from the other elements does not concern us. Now even this seems to have a share in reason, as we said; at any rate in the continent man it obeys reason and presumably in the temperate and brave man it is still more obedient; for in him it speaks, on all matters, with the same voice as reason.
Therefore the irrational element also appears to be two-fold. For the vegetative element in no way shares in reason, but the appetitive and in general the desiring element in a sense shares in it, in so far as it listens to and obeys it; this is the sense in which we speak of paying heed to one's father or one's friends, not that in which we speak of 'accounting’ for a mathematical property. That the irrational element is in some sense persuaded by reason is indicated also by the giving of advice and by all reproof and exhortation.
And if this element also must be said to have reason, that which has reason (as well as that which has not) will be twofold, one subdivision having it in the strict sense and in itself, and the other having a tendency to obey as one does one's father.
