Front Page Titles (by Subject) QUESTION XCVIII.: OF PERJURY. - Aquinas Ethicus: or, the Moral Teaching of St. Thomas, vol. 2 (Summa Theologica - Secunda Secundae Pt.2)
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QUESTION XCVIII.: OF PERJURY. - St. Thomas Aquinas, Aquinas Ethicus: or, the Moral Teaching of St. Thomas, vol. 2 (Summa Theologica - Secunda Secundae Pt.2) 
Aquinas Ethicus: or, the Moral Teaching of St. Thomas. A Translation of the Principal Portions of the Second part of the Summa Theologica, with Notes by Joseph Rickaby, S.J. (London: Burns and Oates, 1892).
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Article II.—Is all perjury a sin?
R. To swear is to call God to witness. Now it is an irreverence to God to call Him to witness to a falsehood, as though God either did not know the truth, or were willing to be a witness to what is false. And therefore perjury is manifestly a sin against religion, the virtue which has for its office to show reverence to God.
§ 1. He who swears to do an unlawful act, in swearing incurs the guilt of perjury for lack of justice.1 If however he does not fulfil his oath, he does not thereby incur the guilt of perjury: because it was not a matter that could fall under oath.
§ 2. He who swears not to enter religion,2 or not to give alms, or anything of that nature, in swearing incurs the guilt of perjury for lack of judgment. And therefore when he goes and does the better thing, it is not perjury, but quite the contrary: for the contrary of what he now does could not be matter of an oath.
§ 4. An oath being a personal action, a newly admitted citizen is not bound on oath to observe what the city has sworn to observe. He is bound however in fidelity to share the burdens and obligations of the city, as he has become a partaker in its advantages. A canon who swears to observe the statutes made in any college, is not bound on oath to observe the statutes to be made hereafter, unless it was his intention to bind himself to all statutes past and to come. He is bound however to observe them by the mere force of the statues themselves, which are compulsory.
Article III.—Is all perjury a mortal sin?
R. According to the doctrine of the Philosopher, “that which makes other things of this or that quality, is itself of the same quality in a higher degree.” But we see that sins of themselves venial, or even actions good of their kind, are mortal sins, if they are done on a motive of contempt of God. Much more therefore is everything a mortal sin, that of its own nature appertains to contempt of God. But perjury of its own nature implies a contempt of God: for this is the element of guilt in it, that it is a piece of irreverence to God. Hence perjury of its own nature is a mortal sin.
§ 1. As was said above, q. 89. art. 7. § 3. compulsion does not take away from a promissory oath its binding power in respect of that which may lawfully be done. And therefore if a party does not keep an oath taken on compulsion, he none the less commits perjury and sins mortally. He may, however, be absolved from the obligation of his oath by the authority of the Sovereign Pontiff, especially if he were constrained by threats, formidable enough to cause fear in a resolute man.
[1 ]See II-II. q. 89. art. 3. (Trl.)
[2 ]See II-II. q. 89. art. 7. § 2. with note. (Trl.)