a. About the First Doubt

14. As to the first of these doubts it is commonly conceded that a single slight perjury is not a mortal sin.

15. However, to be a perjurer habitually is a mortal sin. And this it seems can be proved, because a habit generated by many acts inclines one to a graver act than are the preceding ones.

16. But against this: if the first act of perjury is not a mortal sin, then neither is any other, even from whatever habit it comes to be, because an inclining habit cannot make an act graver. For if someone acquired a great habit from acts of incontinence and quickly repent, then if after repentance he have some movement of incontinence, although his great habit incline him to it, yet it is not a mortal sin in him; indeed neither is it notably graver than it would be in someone else who has no habit.

17. A confirmation is that a habit cannot be graver, if it has any gravity. Since however gravity has no culpability, properly speaking, save from acts, then since the acts from which the habit is generated are venial, the habit will not produce any mortal gravity in acts elicited by the habit.

18. It seems then possible to say that a habit or custom does nothing for the proposed conclusion. But perjury, with full consent, is against the commandment of the first table and consequently turns one aside immediately from the ultimate end, and so nothing of the idea of mortal sin is lacking to it.

19. But if a perjury is not done deliberately, however often it is done, then since a meritorious act requires that it be fully human and so fully deliberate, an act of demerit would require altogether the same (for God is not more prone to punish than to condone) - if so, one can say that a non-deliberate perjury, even repeated ever so often, is not a mortal sin.

20. However, as was said before in the material about the virtues [d.33 n.77], a virtuous man has quick deliberation (which does not seem to be deliberation because of its quickness), because he has great prudence wherewith he is prompt to deliberating as it were in imperceptible time. So too someone could, from a habit opposite to prudence, acquire for himself a facility in deliberating promptly about the opposite, as if in imperceptible time, and this deliberation, following the habit, would be sufficient for the idea of sin; just as the like deliberation in a good man would be sufficient for the idea of merit.

21. Therefore, as concerns the idea of mortal sin, I do not distinguish so much between the rarity or frequency of perjury as between its deliberation or lack of deliberation, so that when deliberation is concurrent it makes mortal sin (and that whether the act is single or customary), and lack of deliberation excuses, whether once or as many times as you like.