Fr Thomas Crean: The teaching of Aquinas on conscience

By Fr Thomas Crean OP

Rome Life Forum, 17 May 2018

In this talk I wish, first, to state briefly what St Thomas Aquinas meant by the term ‘conscience’; and secondly, to describe two ways in which his teaching on conscience is often distorted, and to show the serious consequences of this for the life of the Church today.

First, then, what does St Thomas mean by the term conscience, in Latin, conscientia? Some people have used the word to denote a special power of the soul, whose province would be the making of moral judgements. However, Aquinas did not believe in the existence of such a power; since our intellect is already a power made for apprehending truth, it follows that any kind of truth, whether ‘moral’ or ‘non-moral’, can be an object of the intellect. Thus it is our intellect that sees, for example, both that ‘twice two is four’ and that ‘lying is wrong’. By conscience, St Thomas means a certain class of judgements made by our intellect; namely, the judgements that we make about acts that we have done or are doing or are thinking of doing. By its etymology, he notes, conscientia suggests the application of knowledge to something; and we are said to make a judgement of conscience when we apply our knowledge to our own actions. He distinguishes three cases: either we are simply aware of the fact of having done something; or else we judge about the goodness or evil of something which we have done; or we judge that a possible future act is something we should or should not or may do.

This explains the actions that are commonly attributed to conscience: for it is said to bear witness, about the mere fact of some past actions; to acquit, accuse or even torment us, about the goodness or evil of a past act; and to impel us or restrain us about some future act.[1] Conscience, then, is the judgement that an actual or potential action of mine is or was good or bad, obligatory, forbidden or optional. Finally, he notes that while the word denotes strictly speaking such acts of judgement, it is also, by a natural process, used to denote one of the internal causes from which these acts spring, namely, our habitual tendency to recognise certain basic goods, such as life and existence in society, as goods which are naturally suited to us.

After that brief account of what St Thomas means by the Latin word conscientia, I come now to consider two ways in which his teaching about conscience comes to be distorted, and to show how serious these distortions are for the Church today.

The first distortion of St Thomas’s teaching is to say that he holds that my conscience is an authority, in the sense of something which authorises me to act. On this view, the mere fact that I judge that some possible action of mine would be good to perform gives to me a right to perform it, and gives to other people a duty to allow me to do so. This distorted view was, I am sorry to say, recently expressed in very strong terms by an American archbishop speaking at a university in England. Referring to ‘married couples and families’, he declared without qualification: “Their decisions of conscience represent God’s personal guidance for the particularities of their lives.” In the course of his talk, the same archbishop explicitly identified the voice of conscience with the voice of God for each person.[2]

Other people, without going quite so far as this archbishop, will say that my judgement about the goodness of some future act gives me a right to perform it, and other people a duty to allow me to perform it, provided that any error I may make in my judgement is not the result of my own negligence in seeking the truth.[3]

This common view, that my judgement of conscience authorises action in accordance with it, and makes such action good, arises out of a misinterpretation of St Thomas’s teaching that ‘an erring reason binds’. To say that ‘an erring reason binds’ means that if I mistakenly perceive some action as good and obligatory on me, then I commit a sin by failing to perform it; and likewise, if I mistakenly perceive some action as evil and forbidden, then I commit a sin if I do perform it.  For example, if I suppose, erroneously, that blood transfusions are against the law of God, then I commit a sin by choosing to have a blood transfusion. As St Thomas puts it:

Since the object of the will is something which is proposed by the reason, therefore, from the very fact that a thing is proposed by the reason as evil, the will by tending towards it becomes evil.[4]

In other words, if I choose what I believe to be evil, even if it is in fact good, then insofar as in me lies, I am consenting to evil and hence committing a sin. Again, if I erroneously suppose that it is obligatory on me to make a pilgrimage to Mecca, then I commit a sin if I choose not to go to Mecca in order to save money, since I am deliberately refusing what appears to me as an obligatory good action.

However, the fact that I commit a sin by acting contrary to my judgement about good and evil does not imply that I act well by following my judgement about good and evil, nor does it imply that anyone else has the duty to allow me to follow it. For example, if I suppose erroneously that blood transfusions are against the law of God, and so am ready to allow a child under my care to bleed to death, then I do not act virtuously; and the civil authority, by contrast, acts well by giving the child a blood transfusion against my will. My conscientious objection to blood transfusions, even if very intense, even if I am ready to go to prison to maintain it, does not make it virtuous or even morally permissible for me to refuse a transfusion for myself or for anyone else.

Thus, for St Thomas, the fact that my will is in conformity with my conscience, though a necessary condition for my will to be good, is not a sufficient condition. My judgement of conscience must also be true.

Two things, then, are necessary for an action of mine to be good: I must make a correct judgement of conscience, and I must act in accordance with it. The mere fact that an action is in accordance with my conscience does not authorise me to perform it: this is what I mean by denying that for St Thomas, conscience is of itself an authority. What has authority are God, and the good; conscience only has authority to the extent that it adheres to God and to the good; that is to say, insofar as it judges truly.

This, then, is the first of the two distortions of Aquinas’s teaching that I wished to mention. It is not difficult to see how devastating this distortion is for true authority, both ecclesiastical and civil. If an action were good from the mere fact of its being in conformity with conscience, with no requirement that the judgement of conscience should itself be true, what right would civil and ecclesiastical authority have to prohibit any kind of evil or harmful action, provided that those perpetrating them claimed to be acting in conscience? The suicide bomber who conscientiously carried out his mission would then be performing a good action, and what right has anyone to hinder another’s good deed? Or within the Church, if someone were to feel bound in conscience to live in an adulterous or unnatural union, the pastors would have to allow him, and indeed to praise him, for doing so.

The second distortion of St Thomas’s teaching lies in a misuse of something else which he says about the mistaken conscience. As we have just seen, he holds that no will which accords with a false judgement of conscience can be good. Nevertheless, he also holds that there are circumstances in which such a will may not be bad. It depends on what causes the person to be mistaken in the first place, and, in particular, on whether his ignorance is voluntary. For example, let us suppose that some bottle of medicine has been erroneously labelled by the manufacturer. The chemist who sells it therefore supposes that it contains drugs that will cure a certain patient when in fact it contains drugs that will kill him. The chemist makes the erroneously judgement of conscience, ‘It would be good to give this bottle to this patient’, and acts accordingly.

St Thomas would say that the choice to give the bottle, and the giving of it, are not good; yet neither are they bad. They are the result of an ignorance which was not in any way voluntary on the chemist’s part, and so they are not morally imputable to him either as good or as bad. Thus it is possible, on St Thomas’s principles, erroneously to suppose a certain action to be good, in this case, giving to someone drugs that will kill him, and to perform this action without committing a sin.

On the other hand, if the chemist had known that the manufacturers had labelled bottles incorrectly in the past and did not attempt to verify the contents of the new bottle, then he might well be guilty of negligence, and in that case neither his ignorance about the faulty label nor his decision to hand over the wrong bottle would be free from fault. In general, Aquinas says, where the ignorance at the root of the faulty conscience derives either from negligence in seeking the truth or even from a conscious decision not to seek it, then such ignorance is voluntary, and actions deriving from it are morally imputable to the agent.[5] Where, by contrast, ignorance is involuntary, it is inculpable, and actions that derive from it are not morally imputable.

Now, some authors have suggested that inculpable ignorance can extend not only to knowledge of individual facts, as in the case of the mislabelled bottle, but also to the divine commandments themselves. For example, they argue, someone may judge, erroneously but inculpably, that adultery is not an intrinsic evil. Such a person, reflecting on some possible act of adultery, may thus make the erroneous judgement: ‘This action is permitted to me’. Although, these authors say, any resulting act of adultery would not be good, yet nor would it be imputable to the agent as bad, since his decision that it was permissible resulted from his invincible ignorance about the intrinsic evil of adultery. It might therefore even be possible that he would remain in a state of grace while committing adultery.

In the rest of this talk, I want to consider four slightly different versions of this claim, made today by certain prelates and theologians, that it is possible in virtue of an invincibly ignorant conscience to violate the divine commandments while remaining in a state of grace, and to see how St Thomas would have responded to them. To avoid complications that would otherwise arise, and in order to respond more closely to today’s – shall we say? – ecclesial situation, I shall consider only the case of Catholics who violate divine commandments forbidding intrinsically evil acts.

The hypothesis in question appears to be put forward today in one simple form and in three more complex forms. I shall summarise them and then consider each in turn.

The simple form of the hypothesis claims that Catholics, while being in a state of grace, may be simply ignorant of a divine commandment; for example, that they may be ignorant that God has forbidden fornication, and hence may commit fornication while remaining in a state of sanctifying grace. Thus, a recent article in the worthy French theological journal Revue Thomiste suggests that Catholics today who co-habit outside marriage may be free of the guilt of mortal sin because their lack of religious instruction and the general corruption of the culture makes them unaware of the seriousness of their action.[6]

The second version of the hypothesis is simply a more extreme form of the first. It claims that Catholics, while being in a state of grace, can be ignorant that something is contrary to the law of God even though they know that the Church teaches that it is thus contrary. For example, they may know that the Church teaches that adultery is contrary to the law of God, but they may be blamelessly unable to accept the truth of this teaching (they have a difficulty in grasping ‘the value of the rule’), and hence they may commit adultery while remaining in a state of grace.

The third version of the hypothesis claims that Catholics, while being in a state of grace can know something to be forbidden by the law of God but can be ignorant that this law is seriously binding upon them, rather than binding only under pain of venial sin; or else that they can think of the law as being only a kind of ideal or counsel of perfection. For example, they may know that adultery is contrary to the law of God, but may nonetheless be invincibly ignorant that they are seriously obliged to avoid adultery, and hence may commit this sin while remaining in a state of grace.

The fourth and final version of the hypothesis claims that Catholics may know that a divine commandment is seriously binding, yet may be free of the guilt of mortal sin if they break it, because the only alternatives seem to them to involve committing a worse sin. For example, a Catholic may know that adultery is contrary to the law of God, and know that this law is seriously binding, and yet may commit adultery while remaining in a state of grace because he supposes that his only alternative course is to do something contrary to an even more important divine command.

In what follows, I intend to describe how St Thomas would respond to these four hypotheses.

The first claim, then, was that Catholics in a state of grace may be ignorant of a divine commandment and hence may remain in a state of grace while breaking it. As we have seen, the general principle that St Thomas puts forward about the effect of ignorance on the goodness or badness of an act is that the voluntariness or involuntariness of the ignorance causes the act that is performed in virtue of this ignorance to be itself either voluntary or involuntary, and hence to be either imputable or to the agent or not. The question, therefore, is whether it is possible for a Catholic to be involuntarily ignorant of a divine commandment about intrinsically evil actions.

Now, so far from allowing this possibility to Catholics, St Thomas excludes it from mankind in general. In the Summa Theologiae, question 88, article 6 of the Prima Secundae, he asks whether a sin which is by nature mortal can become venial owing to some circumstance. He will argue that it cannot, but first he imagines an objector putting to him the following case:

It may happen [the objector says] that a person in committing a sin which is of a mortal kind, loves a creature less than God; for instance, if someone who is ignorant that simple fornication is a mortal sin contrary to the love of God, commits the sin of fornication, yet so as to be ready, for the love of God, to refrain from that sin if he knew that by committing it he was acting counter to the love of God. Therefore his will be a venial sin; and accordingly a mortal sin can become venial (obj. 2).

St Thomas replies that in such a case the person committing fornication would still be guilty of a mortal sin, since he had been gravely negligent in ascertaining what law of God was. He writes: “The ignorance itself is a sin, and contains within itself a lack of the love of God, in so far as the man neglects to learn those things whereby he can safeguard himself in the love of God.”

It is noteworthy that Aquinas gives the example of fornication. He was aware that the pagans in general had not considered this to be a grave matter;[7] yet he does not think a person acting in accordance with such an error to be free from mortal sin. Hence, although the article from the Revue Thomiste from which I have quoted is correct to say that the pagan atmosphere of the modern world makes it more likely that Catholics will be ignorant of the divine laws that govern sexual behaviour, St Thomas holds that this ignorance is itself still mortally sinful. It contains within itself, he says, a lack of the love of God, since it means that a person has neglected to learn how to please Him. The bad example of the surrounding culture can certainly lessen the guilt, but it cannot, according to St Thomas, convert it from mortal to venial.

Similarly, and naturally, given that adultery is graver than fornication, he rejects the idea that ignorance of the divine law against adultery can ever be involuntary. So he writes:

If an erring reason tells a man that he should go to another man’s wife, the will that abides by that erring reason is evil; since this error arises from ignorance of the divine law, which he is obliged to know.[8]

We see here, it seems to me, a certain danger in the maxim, often repeated, that ‘full knowledge and consent’ are necessary conditions for mortal sin. It is not in fact necessary to know that something is a mortal sin in order to incur the guilt of mortal sin by committing it. St Thomas’s consistent position is that the only kind of lack of awareness of God’s commandments which can excuse from mortal sin someone who breaks them is a lack of awareness due to madness or mental handicap, things which in his opinion excuse even from venial sin.[9] It is interesting to note that the modern Catechism of the Catholic Church, although it uses the maxim about ‘full knowledge and consent’ as necessary for mortal sin, goes on immediately to add that ‘feigned ignorance and hardness of heart’ do not excuse one, and also that “no one is deemed to be ignorant of the principles of the moral law”.[10] Given these large qualifications, and given the importance of the question at stake, I wonder if it is wise to continue to use this maxim in preaching and catechesis.

The second of the four hypotheses to be considered was that Catholics, while living in grace, can be ignorant that something is contrary to the law of God although they know that the Church teaches that it is; they inculpably fail to see the value of the rule. I don’t want to spend long on this hypothesis, since it is in fact excluded by the same arguments that excluded the first. If men in general are guilty for their ignorance when they do not know that that certain acts are intrinsically evil, then clearly Catholics are too. In fact, St Thomas would hold that such Catholics are not only guilty of culpable ignorance but also of the much graver sin of heresy.

It is manifest [he writes] that he who adheres to the teaching of the Church, as to an infallible rule, assents to whatever the Church teaches; otherwise, if, of the things taught by the Church, he holds what he chooses to hold, and rejects what he chooses to reject, he no longer adheres to the teaching of the Church as to an infallible rule, but to his own will.[11]

A Catholic who refused to accept the Church’s infallible teaching about the divine law would by that fact be unable to possess the virtue of faith, and so could not be living in a state of grace.

The third version of the hypothesis which I have distinguished suggests that Catholics in a state of grace who know that something is forbidden by the law of God can be ignorant that this law binds them gravely, and can suppose that it binds only under pain of venial sin, or even that, like the rules, though not the vows, of religious orders, it does not bind under pain of sin at all, but presents them rather with a kind of ideal at which to aim.

Aquinas, I think, would simply say that this suggestion misunderstands what it is to know, as we are obliged under pain of gravely sinful negligence to know, that something belongs to divine law. To know that something belongs to divine law just is to know that there is a divine commandment about it, and that anyone who breaks this commandment sins mortally by preferring some created thing to God. Hence in the De Veritate, he writes that the very fact that someone has the will not to observe the law of God, means that he sins mortally.[12] In other words, part of knowing that something belongs to divine law is to know that it binds gravely. Someone who thinks that what he may still call ‘divine law’ is in fact only a kind of ideal is therefore in the position considered in the first hypothesis, i.e. such a person is simply ignorant of the divine law about intrinsically evil acts, an ignorance which St Thomas holds to be gravely sinful.

The fourth and final hypothesis that needs to be considered is that a Catholic, while being in a state of grace, may know that a divine commandment is seriously binding on him, but may free of the guilt of mortal sin if he breaks it, because the only alternative course seems to him to involve a worse sin.

This was already a classic question in the 13th century. Technically, such a person was said to be perplexus, ‘perplexed’. St Thomas’s consistent teaching here is that no one need remain in such a state, since it is caused only by some underlying bad choice which can always be revoked. Thus, he says, if someone likes to do what is right in order to be praised by others, such a person might seem to be perplexed, since whether he does the right thing in order to win praise or whether he omits to do it, he will be guilty, either of vainglory or of a sin of omission. Yet such a man, he goes on, is not truly perplexed, since he can always set aside his underlying intention of acquiring human praise and then do the right thing for the right reason.[13] More generally, we can say that it is impossible for a person so to entangle his life that there is no longer any good course left for him to take, and that he must break God’s law in one way or another. To claim that this is possible is to say that God’s law will sometimes make incompatible demands on us, which would be to blaspheme against divine wisdom.

To conclude: as various authors have noted, St Thomas’s moral teaching is not a ‘morality of conscience’, in this sense that he does not hold that good action means action in conformity with conscience.[14] Good action, rather, means action that attains God and the good, with our correct judgement of conscience being simply a necessary condition for this. Far from always being the voice of God, conscience can be the voice of the flesh, or even of the devil. While it is possible for men to be ignorant of divine law and the intrinsic evil of certain kinds of action, such ignorance does not excuse them from mortal sin, since even where bad education or a corrupt ambient culture exist as mitigating factors, their very ignorance is still itself a mortal sin: a negligence, St Thomas says, to learn those things whereby we can safeguard ourselves in the love of God. In this he is true to the teaching of the New Testament. Writing to the Ephesians, St Paul does not say that the ignorance of the divine law under which the Gentiles laboured excused them from guilt, but rather that it cut them off from God; they are, he says, alienated from the life of God through the ignorance that is in them (Eph. 4:18). And the apostle of divine love assures us that it is not possible to combine the state of grace and intrinsically evil actions. Little children, let no one deceive you; he that doth justice is just. . . Whosoever is born of God, committeth not sin: for his seed abideth in him, and he cannot sin, because he is born of God (1 Jn. 3: 7, 9).


[1] The modern Catechism uses the word in a slightly more limited way: ‘Conscience is a judgement […] whereby the human person recognises the moral quality of a given act that he is going to perform, is in the process of performing or has already completed’ (CCC 1778).

[2] Cardinal Blaise Cupich, speaking on 9th February at St Edmund’s College, University of Cambridge. Full text available here: http://www.lastampa.it/2018/02/09/vaticaninsider/eng/documents/pope-francis-revolution-of-mercy-amoris-laetitia-as-a-new-paradigm-of-catholicity-skMox0lKtoX5szfKH6QgrL/pagina.html

[3] E.g. E. D’Arcy, Conscience and Its Right to Freedom, London, Sheed & Ward, 1961, quoted in J. Lamont, ‘Conscience, Freedom, Rights: Idols of the Enlightenment Religion’, The Thomist, , 73 (2009), 182.

[4] Summa Theologiae 1a 2ae, 19, 5.

[5] Ibid.

[6] T. Michelet, ‘La Communion des divorcés remariés’, Revue Thomiste 116, octobre-décembre 2016, t. 4, 633: « Ils vivent dans des conditions culturelles telles qu’ils peuvent fort bien ne pas avoir de péchés graves sur la conscience en ce domaine, dans la mesure où ils n’en voient plus la gravité, aidés en cela par la société, si ce n’est par leurs éducateurs. »

[7] Cf. 1a 2ae 103, 4 ad 3: “With regard to fornication a special prohibition was made, because the Gentiles did not hold it to be sinful.” He is speaking about the Council of Jerusalem, described in Acts 15.

[8] 1a 2ae, 19, 6.

[9] Quodlibetal Questions, III, 12, 2 ad 2: “Ignorance of the law does not excuse one from sin, unless it be invincible ignorance, such as us found in lunatics and imbeciles (furiosis et amentibus), which entirely excuses from sin.”

[10] CCC 1859-60.

[11] 2a 2ae 5, 3.

[12] Cf. De Veritate, 17, 4 : “Non videtur autem possibile quod aliquis peccatum evadat, si conscientia, quantumcumque errans, dictet aliquid esse praeceptum Dei sive sit indifferens sive etiam per se malum; si contrarium, tali conscientia manente, agere disponat. Quantum enim in se est, ex hoc ipso habet voluntatem legem Dei non observandi; unde mortaliter peccat.

[13] 1a 2ae, 19, 6 ad 3.

[14] See J. Lamont’s discussion of the phrase ‘moralities of conscience’ in ‘Conscience, Freedom, Rights’, especially pp. 177-94.