10 As concerning purity of body; here indeed a very honorable regard seems to come in the way, and to demand a lie in its behalf; to wit, that if the assault of the ravisher may be escaped by means of a lie, it is indubitably right to tell it: but to this it may easily be answered, that there is no purity of body except as it depends on integrity of mind; this being broken, the other must needs fall, even though it seem intact; and for this reason it is not to be reckoned among temporal things, as a thing that might be taken away from people against their will.
By no means therefore must the mind corrupt itself by a lie for the sake of its body, which it knows remains incorrupt if from the mind itself incorruptness depart not. For that which by violence, with no lust foregoing, the body suffers, is rather to be called deforcement than corruption. Or if all deforcement is corruption, then not every corruption has turpitude, but only that which lust has procured, or to which lust has consented. Now by how much the mind is more excellent than the body, so much the more heinous is the wickedness if that be corrupted.
There, then, purity can be preserved, because there none but a voluntary corruption can have place. For assuredly if the ravisher assault the body, and there is no escaping him either by contrary force, or by any contrivance or lie, we must needs allow that purity cannot be violated by another's lust. Wherefore, since no man doubts that the mind is better than the body, to integrity of body we ought to prefer integrity of mind, which can be preserved for ever. Now who will say that the mind of him who tells a lie has its integrity?
Indeed lust itself is rightly defined, An appetite of the mind by which to eternal goods any temporal goods whatever are preferred. Therefore no man can prove that it is at any time right to tell a lie, unless he be able to show that any eternal good can be obtained by a lie. But since each man departs from eternity just in so far as he departs from truth, it is most absurd to say, that by departing therefrom it is possible for any man to attain to any good. Else if there be any eternal good which truth comprises not, it will not be a true good, therefore neither will it be good, because it will be false.
But as the mind to the body, so must also truth be preferred to the mind itself, so that the mind should desire it not only more than the body, but even more than its own self. So will the mind be more entire and chaste, when it shall enjoy the immutability of truth rather than its own mutability. Now if Lot, being so righteous a man that he was meet to entertain even Angels, offered his daughters to the lust of the Sodomites, to the intent, that the bodies of women rather than of men might be corrupted by them; how much more diligently and constantly ought the mind's chasteness in the truth to be preserved, seeing it is more truly preferable to its body, than the body of a man to the body of a woman?
Source: On Lying (New Advent)