In his Summa Theologies[1] Aquinas discusses this whole question with eminent fairness, and with great thoroughness.  He first states the claims of those who, from the days of Chrysostom, had made excuses for lying with a good end in view, and then he meets those claims severally.  He looks upon lies as evil in themselves, and as in no way to be deemed good and lawful, since a right concurrence of all elements is essential to a thing’s being good.  “Whence, every lie is a sin, as Augustine says in his book ‘Against Lying.’” His conclusion, in view of all that is to be said on both sides of the question, is:  “Lying is sinful not only as harmful to our neighbor, but because of its own disorderliness.  It is no more permitted to do what is disorderly [that is, contrary to the divine order of the universe] in order to prevent harm, than it is to steal for the purpose of giving alms, except indeed in case of necessity when all things are common property [when, for instance, the taking of needful food in time of a great disaster, as on a wrecked ship, is not stealing].  And therefore it is not allowable to utter a lie with this view, that we may deliver one from some peril.  It is allowable, however, to conceal the truth prudently, by a sort of dissimulation, as Augustine says.”  This recognizes the correctness of Augustine’s position, that concealment of what one has a right to conceal may be right, provided no lie is involved in the concealment.  As to the relative grades of sin in lying, Aquinas counts lying to another’s hurt as a mortal sin, and lying to avert harm from another as a venial sin; but he sees that both are sins.

[Footnote 1:  Secunda Secundae, Quaestio CX., art.  III.]

It is natural to find Aquinas, as a representative of the keen-minded Dominicans, standing by truth as an eternal principle, regardless of consequences; as it is also natural to find, on the other side, Duns Scotus, as a representative of the easy-going Franciscans, with his denial of good absolute save as manifested in the arbitrary will of God.  Duns Scotus accepted the “theory of a twofold truth,” ascribed to Averroes, “that one and the same affirmation might be theologically true and philosophically false, and vice versa.”  In Duns Scotus’s view, “God does not choose a thing because it is good, but the thing chosen is good because God chooses it;” “it is good simply and solely because God has willed it precisely so; but he might just as readily have willed the opposite thereof.  Hence also God is not [eternally] bound by his commands, and he can in fact annul them."[1] According to this view, God could forbid lying to-day and justify it to-morrow.  It is not surprising, therefore, that “falsehood and misrepresentation” are “under certain circumstances allowable,” in the opinion of Duns Scotus.

[Footnote 1:  See Kurtz’s Church History (Macpherson’s Translation), II., 101, 167-169; Ueberweg’s History of Philosophy, I., 416, 456 f.; Wuttke’s Christian Ethics (Am. ed.), I., 218, Sec. 34.]