40 Now the things which are to be kept safe for sanctity's sake are these: pudicity of body, and chastity of soul, and verity of doctrine. Pudicity of body, without consent and permission of the soul, does no man violate. For, whatever against our will and without our empowering the same is by greater force done upon our body, is no lewdness. Howbeit, of permitting there may be some reason, but of consenting, none. For we consent, when we approve and wish: but we permit even not willing, because of some greater turpitude to be eschewed.
Consent, truly, to corporal lewdness violates also chastity of mind. For the mind's chastity consists in a good will and sincere love, which is not corrupted, unless when we love and desire that which Truth teaches ought not to be loved and desired. We have therefore to guard the sincerity of love toward God and our neighbor; for in this is chastity of mind sanctified: and we must endeavor with all the strength in our power, and with pious supplication, that, when the pudicity of our body is sought to be violated, not even that outermost sense of the soul, which is entangled with the flesh, may be touched with any delight; but if it cannot this, at least the mind and thought in not consenting may have its chastity preserved entire.
Now what we have to guard in chastity of mind, is, as pertaining to the love of our neighbor, innocence and benevolence; as pertaining to the love of God, piety. Innocence is that we hurt no man; benevolence, that we also do good to whom we can; piety, that we worship God. But as for verity of doctrine, of religion and piety, that is not violated unless by a lie; whereas the highest and inmost Verity Itself, Whose that doctrine is, can in no way be violated: which Truth to attain unto, and in It on every wise to remain, and to It thoroughly to cleave, will not be permitted, but when this corruptible shall have put on incorruption, and this mortal shall have put on immortality.
But, because all piety in this life is practice by which we tend to that life, which practice has a guidance afforded unto it from that doctrine, which in human words and signs of corporal sacraments does insinuate and intimate Truth herself: for this cause this also, which by lying is possible to be corrupted, is most of all to be kept incorrupt; that so, if anything in that chastity of mind be violated, it may have that wherefrom it may be repaired. For once corrupt authority of doctrine, and there can be none either course or recourse to chastity of mind.
Source: On Lying (New Advent)