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 that speaks the truth, thinking it to be false, lies, according to Augustine (Lib. De Mendac. iii). Therefore lying is not opposed to truth.
Objection 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.
Objection 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, 18, 6,7).
Now it has been said above (109, 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.
Accordingly if these three things concur, namely, falsehood of what is said, the will to tell a falsehood, and finally the intention to deceive, then there is falsehood--materially, since what is said is false, formally, on account of the will to tell an untruth, and effectively, on account of the will to impart a falsehood.
However, the essential notion of a lie is taken from formal falsehood, from the fact namely, that a person intends to say what is false; wherefore also the word "mendacium" [lie] is derived from its being in opposition to the "mind." Consequently if one says what is false, thinking it to be true, it is false materially, but not formally, because the falseness is beside the intention of the speaker so that it is not a perfect lie, since what is beside the speaker's intention is accidental for which reason it cannot be a specific difference. If, on the other hand, one utters' falsehood formally, through having the will to deceive, even if what one says be true, yet inasmuch as this is a voluntary and moral act, it contains falseness essentially and truth accidentally, and attains the specific nature of a lie.
That a person intends to cause another to have a false opinion, by deceiving him, does not belong to the species of lying, but to perfection thereof, even as in the physical order, a thing acquires its species if it has its form, even though the form's effect be lacking; for instance a heavy body which is held up aloft by force, lest it come down in accordance with the exigency of its form. Therefore it is evident that lying is directly an formally opposed to the virtue of truth.
Reply to Objection 1. We judge of a thing according to what is in it formally and essentially rather than according to what is in it materially and accidentally. Hence it is more in opposition to truth, considered as a moral virtue, to tell the truth with the intention of telling a falsehood than to tell a falsehood with the intention of telling the truth.
Reply to Objection 2. As Augustine says (De Doctr. Christ. ii), words hold the chief place among other signs. And so when it is said that "a lie is a false signification by words," the term "words" denotes every kind of sign. Wherefore if a person intended to signify something false by means of signs, he would not be excused from lying.
Reply to Objection 3. The desire to deceive belongs to the perfection of lying, but not to its species, as neither does any effect belong to the species of its cause.
Source: Summa Theologica (New Advent)