Discussion of Conscience

Conscience, St. Thomas says, is knowledge - particularly as knowledge is applied to the question of what to do or to the judgment of what has been done:

According to the other way in which knowledge is applied to an act such that it is known whether it is right, there is a twofold path. One according as through the habit of science we are directed that we should or should not do something; another insofar as the act once done is examined by the habit of science as to whether it was right or not. ... When knowledge is applied to an act in directing it, conscience is said to goad or persuade or bind; when knowledge is applied to an act in the manner of an examination of what has already been done, conscience is said to accuse or worry when what was done is found to be out of harmony with the knowledge by which it is examined, or to defend or excuse when what has been done is found to have taken place in conformity with the science (Disputed Question on Truth 17: On Conscience, Article 1, Response: p. 222)

The fact that conscience is knowledge, and that our knowledge is limited and prone to error, means that conscience can err (Article 2). As a consequence, it cannot be said that conscience is an infallible guide to conduct. Aquinas doesn't go on to discuss the implications of this for apologetics (!) but it may be seen that those who exalt their own consciences over against the infallible teaching of the Church on questions of faith and morals are in error. Nevertheless, even an erroneous conscience binds:

"Moreover, according to Damascene, conscience is the 'law of our intellect.' But to act against the law is a sin. Therefore in whatever way a conscience be erroneous, it binds" (Article 4, sed contra: ibid., p. 233).

But it seems that this means a man does not sin if he follows his erroneous conscience. Can a man have the erroneous belief that it is right to commit adultery with this woman, and in so doing avoid sinning? No, says St. Thomas:

But those who say [this sort of thing] seem not to understand what it is for conscience to bind. That conscience binds means that when when does not follow it he incurs sin, not that one following it does the right thing. ... Therefore conscience is not said to oblige us to do something because to follow it is good but because not to follow entails sin. ... A correct and erroneous conscience binds differently, however: a correct one binds absolutely and as such, whereas an erroneous one does so accidentally and in a certain respect. ...

For example, he who loves wine for the sake of its sweetness, loves sweetness per se and wine accidentally. He who has an erroneous conscience, believing it to be correct -- otherwise he would not err -- adheres to the erroneous conscience on account of the rectitude he thinks it to have; he adheres then per se speaking to a correct conscience and to the erroneous one as it were accidentally, insofar as the conscience he believes to be correct is erroneous (ibid., p. 233f.; emphasis added).

If, through an erroneous conscience one believes that he must do that which is contrary to the Law of God, he is obliged to follow his conscience, but he sins mortally in doing so, "since it came about through ignorance of that which he ought to know" (ibid., p. 235).

It would seem on this account that sometimes men cannot avoid sin: if their conscience is wrong, they may sin by obeying it, or they may sin by not following it. To this Aquinas says, "it is not absurd to say that, something being supposed, a man cannot avoid sin. For example, presupposing the intention of vainglory, he who is held to give alms cannot avoid sin, for if he gives with this intention, he sins, and if he does not give, he is a transgressor" (A4 ad 8, p. 236). So, presuming a culpable ignorance of the law, a man may not be able to avoid sin.