Aquinas regards the souls of men, like the angels, as immaterial forms; and he includes in the soul-unit, so to speak, not merely the anima rationalis of Aristotle, but also the vegetative, sensitive, appetitive and motive functions.
Happiness in this world consists proximately in virtue as a harmony between the three parts, rational, spirited and appetitive, of our souls, and ultimately in living according to the form of the good; but there is a far higher happiness, when the immortal soul, divesting itself of body and passions and senses, rises from earth to heaven and contemplates pure forms by pure reason.
These non-rational elements he further distinguished as appetitive (ro E7rLOvi..arr KOv) and spirited (TO Ov,uoabS or Ovu6r) - the practical separateness of which from each other and from reason he held to be established by our inner experience.
In the first place, though in Aristotle's view the most perfect well-being consists in the exercise of man's " divinest part," pure speculative reason, he keeps far from the paradox of putting forward this and nothing else as human good; so far, indeed, that the greater part of his treatise is occupied with an exposition of the inferior good which is realized in practical life when the appetitive or impulsive (semi-rational) element of the soul operates under the due regulation of reason.
Until this is done, the utmost demonstration of the abstract reasonableness of social duty only leaves us with an irreconcilable antagonism between the view of abstract reason and the self-love which is allowed to be the root of man's appetitive nature.