The answer to this question is that one is obliged to do what one believes is right only if it is objectively right. If it is objectively wrong, one is not only not obliged to do it, but is obliged not to do it. Of course, most people will think this claim nonsensical. The automatic, common-sense view is that because one can only act with moral freedom, whether in mind or physically, on one’s perceptions of right and wrong, be they right perceptions or wrong ones, it is senseless to say that one can be obliged to do something, or not do something, if through no fault of one’s own one is unaware of the truth. However, in this instance common sense is in error.

Look at the matter this way. If you think something is right or wrong, you necessarily think it is so irrespective of your reasoning process. You necessarily, even if only implicitly and unconsciously, regard your reasoning process merely as an instrument for finding the truth – and a fallible instrument. After all, your fundamental purpose in reasoning is to reach right conclusions and avoid wrong ones, which means that you necessarily recognise, even if (God forbid) you prattle the contrary, that there is reality outside your reasoning by which the reasoning and its conclusions can be identified as right or wrong. If there were no such outside reality, your conclusions could never be either right or wrong, but could only ever be, like dreams, mere facts which do not admit or moral categorisation.

Let us apply this logic specifically to reasoning about positive and negative moral obligations – “must do” and “must not do” actions (including actions of mind which are subject to the will). In order to reach a conclusion on whether some action, or contemplated action, is obligatory, permissible or forbidden, one must assume that it is so regardless of one’s thinking. This means accepting that if one reasons wrongly, and mistakenly concludes that an action which is morally forbidden is permissible, the wrong reasoning does not render the action obligatory or permissible for oneself – it remains forbidden to one, and one still does wrong if one performs it.

However, doing right or wrong is one thing, but being morally responsible for doing so – being praiseworthy for doing right, or culpable for doing wrong – is another. A person who does wrong in invincible ignorance of its wrongfulness is non-culpable for his deed; and a person who does right thinking it is wrong is not only not praiseworthy for the deed, but is guilty of the incidental, culpable evil of willing to do wrong in general. This all boils down to the following. Strictly speaking, one is never obliged or permitted to follow a wrong conscience, but is always forbidden to follow it; yet because the possessor of a wrong conscience is by definition unaware of its wrongfulness, he is morally culpable if he does not follow it, and morally non-culpable – even praiseworthy – if he does follow it. It was never morally permissible, let alone morally obligatory, for any German soldier to murder a Jew, even if his mind and conscience had been so addled that he was convinced it was his duty to do so; and yet if a soldier who was thus convinced acted on his conviction, he might not have been fully morally culpable for his crime.

Therefore, given that conscience is fallible, the only sense in which the statement that one is always obliged to follow one’s conscience can be true is a loose one – just as the statement that in matters of love one should follow one’s heart can be true if meant loosely, but is neither true nor possible if meant strictly. (I might add that Aquinas in his Summa Contra Gentiles was wrong on the question of whether a wrong conscience binds, but had rectified his views and was right on the question in his subsequent and greatest work, the Summa Theologica – although there he still struggles to make everything he says which pertains to the issue fully mutually consistent.)