Thomas Aquinas

“ Now it is a mortal sin to act against a precept. Therefore every lie of the perfect is a mortal sin: and consequently so also is a lie told by anyone else, otherwise the perfect would be worse off than others.
On the contrary, Augustine says on Ps. 5:7, "Thou wilt destroy," etc.: "There are two kinds of lie, that are not grievously sinful yet are not devoid of sin, when we lie either in joking, or for the sake of our neighbor's good." But every mortal sin is grievous. Therefore jocose and officious lies are not mortal sins. ”

“ Now the sin of lying is aggravated, if by lying a person intends to injure another, and this is called a "mischievous" lie, while the sin of lying is diminished if it be directed to some good---either of pleasure and then it is a "jocose" lie, or of usefulness, and then we have the "officious" lie, whereby it is intended to help another person, or to save him from being injured. In this way lies are divided into the three kinds aforesaid. ”

“ Yet less harm is done by raising a false opinion in a person's mind, than by someone slaying or being slain. Therefore a man may lawfully lie, to save another from committing murder, or another from being killed. ”

“ 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. ”

“ 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.
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. ”

“ 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 ”

“ It is unlawful to hold that any false assertion is contained either in the Gospel or in any canonical Scripture, or that the writers thereof have told untruths, because faith would be deprived of its certitude which is based on the authority of Holy Writ. ”