34 For what is written in another place, “Wish not to use every lie;” they say is not of force for this, that a person is not to use any lie. Therefore, when one man shall say, that according to this testimony of Scripture we must to that degree hold every sort and kind of lie in detestation, that even if a man wish to lie, yea, though he lie not, the very wish is to be condemned; and to this sense interprets, that it is not said, Do not use every lie, but, “Do not wish to use every lie;” that one must not dare not only to tell, but not even to wish to tell, any lie whatever: says another man, “Nay, in that it says, Do not wish to use every lie, it wills that from the mouth of the heart we exterminate and estrange lying: so that while from some lies we must abstain with the mouth of the body, as are those chiefly which pertain to doctrine of religion; from some, we are not to abstain with the mouth of the body, if reason of avoiding a greater evil require; but with the mouth of the heart we must abstain utterly from every lie.”
Where it behooves to be understood what is said, “Do not wish:” namely, the will itself is taken as it were the mouth of the heart, so that it concerns not the mouth of the heart when in shunning a greater evil we lie unwillingly. There is also a third sense in which you may so take this word, “not every,” that, except some lies, it gives you leave to lie. Like as if he should say, wish not to believe every man: he would not mean to advise that none should be believed; but that not all, some however, should be believed.
And that which follows, “For assiduity thereof will not profit for good,” sounds as if, not lying, but assiduous lying, that is, the custom and love of lying, should seem to be that which he would prohibit. To which that person will assuredly slide down, who either shall think that every lie may be boldly used (for so he will shun not that even which is committed in the doctrine of piety and religion; than which what more abominably wicked thing can you easily find, not among all lies, but among all sins?) or to some lie (no matter how easy, how harmless,) shall accommodate the inclination of the will; so as to lie, not unwillingly for the sake of escaping a greater evil, but willingly and with liking.
So, seeing there be three things which may be understood in this sentence, either “Every lie, not only tell not, but do not even wish to tell:” or, “Do not wish, but even unwillingly tell a lie when anything worse is to be avoided:” or, “Not every,” to wit, that except some lies, the rest are admitted: one of these is found to make for those who hold that one is never to lie, two for those who think that sometimes one may tell a lie. But yet what follows, “For assiduity thereof will not profit to good,” I know not whether it can countenance the first sentence of these three; except haply so, that while it is a precept for the perfect not only not to lie, but not even to wish; assiduity of lying is not permitted even to beginners.
As if, namely, on laying down the rule at no time whatever not merely to lie but so much as to have a wish to lie, and this being gainsaid by examples, in regard that there are some lies which have been even approved by great authority, it should be rejoined that those indeed are lies of beginners, which have, in regard of this life, some kind of duty of mercy; and yet to that degree is every lie evil, and by perfect and spiritual minds in every way to be eschewed, that not even beginners are permitted to have assiduous custom thereof.
For we have already spoken concerning the Egyptian midwives, that it is in respect of the promise of growth and proficiency to better things that they while lying are spoken of with approval: because it is some step towards loving the true and eternal saving of the soul, when a person does mercifully for the saving of any man's albeit mortal life even tell a lie.
Source: On Lying (New Advent)