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 Obj. 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 Obj. 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 Obj. 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. _______________________
SECOND ARTICLE [II-II, Q. 110, Art. 2]
Whether Lies Are Sufficiently Divided into Officious, Jocose, and
Mischievous Lies?
Objection 1: It seems that lies are not sufficiently divided into "officious," "jocose" and "mischievous" lies. For a division should be made according to that which pertains to a thing by reason of its nature, as the Philosopher states (Metaph. vii, text. 43; De Part. Animal i, 3). But seemingly the intention of the effect resulting from a moral act is something beside and accidental to the species of that act, so that an indefinite number of effects can result from one act. Now this division is made according to the intention of the effect: for a "jocose" lie is told in order to make fun, an "officious" lie for some useful purpose, and a "mischievous" lie in order to injure someone. Therefore lies are unfittingly divided in this way.
Obj. 2: Further, Augustine (Contra Mendac. xiv) gives eight kinds of lies. The first is "in religious doctrine"; the second is "a lie that profits no one and injures someone"; the third "profits one party so as to injure another"; the fourth is "told out of mere lust of lying and deceiving"; the fifth is "told out of the desire to please"; the sixth "injures no one, and profits someone in saving his money"; the seventh "injures no one and profits someone in saving him from death"; the eighth "injures no one, and profits someone in saving him from defilement of the body." Therefore it seems that the first division of lies is insufficient.