41 There results then from all these this sentence, that a lie which does not violate the doctrine of piety, nor piety itself, nor innocence, nor benevolence, may on behalf of pudicity of body be admitted. And yet if any man should propose to himself so to love truth, not only that which consists in contemplation, but also in uttering the true thing, which each in its own kind of things is true, and no otherwise to bring forth with the mouth of the body his thought than in the mind it is conceived and beheld; so that he should prize the beauty of truth-telling honesty, not only above gold and silver and jewels and pleasant lands, but above this temporal life itself altogether and every good thing of the body, I know not whether any could wisely say that that man errs.
And if he should prefer this and prize it more than all that himself has of such things; rightly also would he prefer it to the temporal things of other men, whom by his innocence and benevolence he was bound to keep and to help. For he would love perfect faith, not only of believing aright those things which by an excellent authority and worthy of faith should to himself be spoken, but also of faithfully uttering what himself should judge right to be spoken, and should speak.
For faith has its name in the Latin tongue, from that the thing is done which is said: and thus it is manifest that one does not exhibit when telling a lie. And even if this faith be less violated, when one lies in such sort that he is believed to no inconvenience and no pernicious hurt, with added intention moreover of guarding either one's life or corporal purity; yet violated it is, and a thing is violated which ought to be kept safe in chastity and sanctity of mind. Whence we are constrained, not by opinion of men, which for the most part is in error, but by truth itself, truth which is eminent above all, and alone is most invincible, to prefer even to purity of body, perfect faith.
For chastity of mind is, love well ordered, which does not place the greater below the smaller. Now it is less, whatever in the body than whatever in the mind can be violated. For assuredly when for corporal chasteness a man tells a lie, he sees indeed that his body is threatened with corruption, not from his own, but from another's lust, but is cautious lest by permitting at least, he be a party. That permission, however, where is it but in the mind? So then, even corporal chasteness cannot be corrupted but in the mind; which not consenting nor permitting, it can by no means be rightly said that corporal chasteness is violated whatever in the body be perpetrated by another's lust.
Whence it is gathered, that much more must the chastity of the mind be preserved in the mind, in the which is the guardianship of the pudicity of the body. Wherefore, what in us lies, both the one and the other must by holy manners and conversation be walled and hedged round, lest from another quarter it be violated. But when both cannot be, which is to be slighted in comparison of which, who does not see? When he sees which to which is to be preferred, the mind to the body, or the body to the mind; and which is more to be shunned among sins, the permitting of another's deed, or the committing of the deed yourself.
Source: On Lying (New Advent)