Objection 1. It would seem that life is not fittingly divided into active and contemplative. For the soul is the principle of life by its essence: since the Philosopher says (De Anima ii, 4) that "in living things to live is to be." Now the soul is the principle of action and contemplation by its powers. Therefore it would seem that life is not fittingly divided into active and contemplative.
Objection 2. Further, the division of that which comes afterwards is unfittingly applied to that which comes first. Now active and contemplative, or "speculative" and "practical," are differences of the intellect (De Anima iii, 10); while "to live" comes before "to understand," since "to live" comes first to living things through the vegetative soul, as the Philosopher states (De Anima ii, 4). Therefore life is unfittingly divided into active and contemplative.
Objection 3. Further, the word "life" implies movement, according to Dionysius (Div. Nom. vi): whereas contemplation consists rather in rest, according to (Wisdom 8:1)6: "When I enter into my house, I shall repose myself with her." Therefore it would seem that life is unfittingly divided into active and contemplative.
On the contrary, Gregory says (Hom. xiv super Ezech.): "There is a twofold life wherein Almighty God instructs us by His holy word, the active life and the contemplative."
I answer that, Properly speaking, those things are said to live whose movement or operation is from within themselves. Now that which is proper to a thing and to which it is most inclined is that which is most becoming to it from itself; wherefore every living thing gives proof of its life by that operation which is most proper to it, and to which it is most inclined. Thus the life of plants is said to consist in nourishment and generation; the life of animals in sensation and movement; and the life of men in their understanding and acting according to reason. Wherefore also in men the life of every man would seem to be that wherein he delights most, and on which he is most intent; thus especially does he wish "to associate with his friends" (Ethic. ix, 12).
Accordingly since certain men are especially intent on the contemplation of truth, while others are especially intent on external actions, it follows that man's life is fittingly divided into active and contemplative.
Reply to Objection 1. Each thing's proper form that makes it actually "to be" is properly that thing's principle of operation. Hence "to live" is, in living things, "to be," because living things through having "being" from their form, act in such and such a way.
Reply to Objection 2. Life in general is not divided into active and contemplative, but the life of man, who derives his species from having an intellect, wherefore the same division applies to intellect and human life.
Reply to Objection 3. It is true that contemplation enjoys rest from external movements. Nevertheless to contemplate is itself a movement of the intellect, in so far as every operation is described as a movement; in which sense the Philosopher says (De Anima iii, 7) that sensation and understanding are movements of a kind, in so far as movement is defined "the act of a perfect thing." On this way Dionysius (Div. Nom. iv) ascribes three movements to the soul in contemplation, namely, "straight," "circular," and "oblique" [Cf. 180, 6].
Source: Summa Theologica (New Advent)