27 Sin, then, is any transgression in deed, or word, or desire, of the eternal law. And the eternal law is the divine order or will of God, which requires the preservation of natural order, and forbids the breach of it. But what is this natural order in man? Man, we know, consists of soul and body; but so does a beast. Again, it is plain that in the order of nature the soul is superior to the body. Moreover, in the soul of man there is reason, which is not in a beast.
Therefore, as the soul is superior to the body, so in the soul itself the reason is superior by the law of nature to the other parts which are found also in beasts; and in reason itself, which is partly contemplation and partly action, contemplation is unquestionably the superior part. The object of contemplation is the image of God, by which we are renewed through faith to sight. Rational action ought therefore to be subject to the control of contemplation, which is exercised through faith while we are absent from the Lord, as it will be hereafter through sight, when we shall be like Him, for we shall see Him as He is. Then in a spiritual body we shall by His grace be made equal to angels, when we put on the garment of immortality and incorruption, with which this mortal and corruptible shall be clothed, that death may be swallowed up of victory, when righteousness is perfected through grace.
For the holy and lofty angels have also their contemplation and action. They require of themselves the performance of the commands of Him whom they contemplate, whose eternal government they freely because sweetly obey. We, on the other hand, whose body is dead because of sin, till God quicken also our mortal bodies by His Spirit dwelling in us, live righteously in our feeble measure, according to the eternal law in which the law of nature is preserved, when we live by that faith unfeigned which works by love, having in a good conscience a hope of immortality and incorruption laid up in heaven, and of the perfecting of righteousness to the measure of an inexpressible satisfaction, for which in our pilgrimage we must hunger and thirst, while we walk by faith and not by sight.
Source: Reply to Faustus the Manichaean (New Advent)