(3) Whether lying is always a sin?
(4) Whether it is always a mortal sin? _______________________
FIRST ARTICLE [II-II, Q. 110, Art. 1]
Whether Lying Is Always Opposed to Truth?
Objection 1: It seems that lying is not always opposed to truth. For opposites are incompatible with one another. But lying is compatible with truth, since he that speaks the truth, thinking it to be false, lies, according to Augustine (Lib. De Mendac. iii). Therefore lying is not opposed to truth.
Obj. 2: Further, the virtue of truth applies not only to words but also to deeds, since according to the Philosopher (Ethic. iv, 7) by this virtue one tells the truth both in one's speech and in one's life. But lying applies only to words, for Augustine says (Contra Mend. xii) that "a lie is a false signification by words." Accordingly, it seems that lying is not directly opposed to the virtue of truth.
Obj. 3: Further, Augustine says (Lib. De Mendac. iii) that the "liar's sin is the desire to deceive." But this is not opposed to truth, but rather to benevolence or justice. Therefore lying is not opposed to truth.
On the contrary, Augustine says (Contra Mend. x): "Let no one doubt that it is a lie to tell a falsehood in order to deceive. Wherefore a false statement uttered with intent to deceive is a manifest lie." But this is opposed to truth. Therefore lying is opposed to truth.
I answer that, A moral act takes its species from two things, its object, and its end: for the end is the object of the will, which is the first mover in moral acts. And the power moved by the will has its own object, which is the proximate object of the voluntary act, and stands in relation to the will's act towards the end, as material to formal, as stated above (I-II, Q. 18, AA. 6, 7).
Now it has been said above (Q. 109, A. 1, ad 3) that the virtue of truth—and consequently the opposite vices—regards a manifestation made by certain signs: and this manifestation or statement is an act of reason comparing sign with the thing signified; because every representation consists in comparison, which is the proper act of the reason. Wherefore though dumb animals manifest something, yet they do not intend to manifest anything: but they do something by natural instinct, and a manifestation is the result. But when this manifestation or statement is a moral act, it must needs be voluntary, and dependent on the intention of the will. Now the proper object of a manifestation or statement is the true or the false. And the intention of a bad will may bear on two things: one of which is that a falsehood may be told; while the other is the proper effect of a false statement, namely, that someone may be deceived.