Summa summae S. Thomae, Tomus II

I say 1°: To act against conscience, whether right or erroneous, is not of itself a special sin; because the precept of acting according to conscience is not a special precept, but general, such as the precept of obeying the law. Whence that it be against right conscience, e.g., to steal, conscience remorsing, it retains the species of the sin committed, namely theft; but that it be against erroneous conscience pertains to that species of sin which conscience dictates against which one acts ¹ : as, if conscience dictate that one must lie to save the life of his neighbor, not lying pertains to sin of homicide; if it dictate that to swear would be sacrilege, if he swear it will be sacrilege. The reason is, because an act is specified by its object, not as it is in itself, but as it is in apprehension.

I say 2°: What is done against conscience, whether vincibly or invincibly erring, is mortal or venial; because as species so also quantity of malice is taken from the object, not according to itself, but as apprehended and proposed to the will ². Whence whoever acts against conscience dictating something to be mortal sins mortally; if conscience dictate it venial he sins venially. Except, unless what conscience vincibly erring dictates venial or light be in reality grave and mortal; because voluntary error, such as vincible is, does not take away essential gravity of sin. Whence whoever steals gold thinking vincibly it to be venial sin nevertheless sins mortally.

I say 3°: If erring conscience dictates something to be evil, but determines it not to be mortal or venial — which happens not rarely — a distinction must be made. Either conscience proposes, at least confusedly, the matter about which it is concerned as something grave or as something light: if the first, to act against it will be mortal; if the second, venial. Always the same reason holds, because quantity of malice of will is from the object apprehended through reason. But if in no way, nor even confusedly, it advert to gravity or lightness, it seems more probable that then to act against it is mortal sin; because thus the agent exposes himself to peril of sinning mortally, and wills to commit the sin whatsoever it be. — It is confirmed. Whoever is ready to swear anything whatsoever, whether true or false, if not advertent to true or false he swear, sins mortally by swearing, even though he swear truth. Ergo similarly.

Moreover for some modification of this resolution, for tranquillity of consciences, we judge with Sylvius, that if anyone were so disposed in mind, that if there came into his mind what he does to be mortal, he would nowise will to commit it, he would be excused from mortal; because from that habitual disposition it is presumed that he does not apprehend as mortal what he does.