Objection 1. It would seem that the will is not evil when it is at variance with erring reason. Because the reason is the rule of the human will, in so far as it is derived from the eternal law, as stated above (Article 4). But erring reason is not derived from the eternal law. Therefore erring reason is not the rule of the human will. Therefore the will is not evil, if it be at variance with erring reason.
Objection 2. Further, according to Augustine, the command of a lower authority does not bind if it be contrary to the command of a higher authority: for instance, if a provincial governor command something that is forbidden by the emperor. But erring reason sometimes proposes what is against the command of a higher power, namely, God Whose power is supreme. Therefore the decision of an erring reason does not bind. Consequently the will is not evil if it be at variance with erring reason.
Objection 3. Further, every evil will is reducible to some species of malice. But the will that is at variance with erring reason is not reducible to some species of malice. For instance, if a man's reason err in telling him to commit fornication, his will in not willing to do so, cannot be reduced to any species of malice. Therefore the will is not evil when it is at variance with erring reason.
On the contrary, As stated in the I, 79, 13, conscience is nothing else than the application of knowledge to some action. Now knowledge is in the reason. Therefore when the will is at variance with erring reason, it is against conscience. But every such will is evil; for it is written (Romans 14:23): "All that is not of faith"--i.e. all that is against conscience--"is sin." Therefore the will is evil when it is at variance with erring reason.
I answer that, Since conscience is a kind of dictate of the reason (for it is an application of knowledge to action, as was stated in the I, 19, 13), to inquire whether the will is evil when it is at variance with erring reason, is the same as to inquire "whether an erring conscience binds." On this matter, some distinguished three kinds of actions: for some are good generically; some are indifferent; some are evil generically. And they say that if reason or conscience tell us to do something which is good generically, there is no error: and in like manner if it tell us not to do something which is evil generically; since it is the same reason that prescribes what is good and forbids what is evil. On the other hand if a man's reason or conscience tells him that he is bound by precept to do what is evil in itself; or that what is good in itself, is forbidden, then his reason or conscience errs. In like manner if a man's reason or conscience tell him, that what is indifferent in itself, for instance to raise a straw from the ground, is forbidden or commanded, his reason or conscience errs. They say, therefore, that reason or conscience when erring in matters of indifference, either by commanding or by forbidding them, binds: so that the will which is at variance with that erring reason is evil and sinful. But they say that when reason or conscience errs in commanding what is evil in itself, or in forbidding what is good in itself and necessary for salvation, it does not bind; wherefore in such cases the will which is at variance with erring reason or conscience is not evil.
But this is unreasonable. For in matters of indifference, the will that is at variance with erring reason or conscience, is evil in some way on account of the object, on which the goodness or malice of the will depends; not indeed on account of the object according as it is in its own nature; but according as it is accidentally apprehended by reason as something evil to do or to avoid. And since the object of the will is that which is proposed by the reason, as stated above (Article 3), from the very fact that a thing is proposed by the reason as being evil, the will by tending thereto becomes evil. And this is the case not only in indifferent matters, but also in those that are good or evil in themselves. For not only indifferent matters can received the character of goodness or malice accidentally; but also that which is good, can receive the character of evil, or that which is evil, can receive the character of goodness, on account of the reason apprehending it as such. For instance, to refrain from fornication is good: yet the will does not tend to this good except in so far as it is proposed by the reason. If, therefore, the erring reason propose it as an evil, the will tends to it as to something evil. Consequently the will is evil, because it wills evil, not indeed that which is evil in itself, but that which is evil accidentally, through being apprehended as such by the reason. In like manner, to believe in Christ is good in itself, and necessary for salvation: but the will does not tend thereto, except inasmuch as it is proposed by the reason. Consequently if it be proposed by the reason as something evil, the will tends to it as to something evil: not as if it were evil in itself, but because it is evil accidentally, through the apprehension of the reason. Hence the Philosopher says (Ethic. vii, 9) that "properly speaking the incontinent man is one who does not follow right reason; but accidentally, he is also one who does not follow false reason." We must therefore conclude that, absolutely speaking, every will at variance with reason, whether right or erring, is always evil.
Reply to Objection 1. Although the judgment of an erring reason is not derived from God, yet the erring reason puts forward its judgment as being true, and consequently as being derived from God, from Whom is all truth.
Reply to Objection 2. The saying of Augustine holds good when it is known that the inferior authority prescribes something contrary to the command of the higher authority. But if a man were to believe the command of the proconsul to be the command of the emperor, in scorning the command of the proconsul he would scorn the command of the emperor. In like manner if a man were to know that human reason was dictating something contrary to God's commandment, he would not be bound to abide by reason: but then reason would not be entirely erroneous. But when erring reason proposes something as being commanded by God, then to scorn the dictate of reason is to scorn the commandment of God.
Reply to Objection 3. Whenever reason apprehends something as evil, it apprehends it under some species of evil; for instance, as being something contrary to a divine precept, or as giving scandal, or for some such like reason. And then that evil is reduced to that species of malice.
Source: Summa Theologica (New Advent)