A Thomist’s guide to lying

Posted on: 25th January 2022  |
Author: Joe Egerton
Category: Theology, philosophy and ethics
Tags: Aquinas

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Against a background of concern about standards of integrity and honesty in public life, Joe Egerton marks the feast of St Thomas Aquinas on 28 January by reflecting on what we might learn from his discussion of the virtue of truthfulness and the vices opposed to it.


St Thomas’s response to the question of ‘whether a lie is always a sin’[1] is direct: ‘every lie is a sin’.[2] The prohibition on lying is exceptionless. Jesuit philosopher Francisco Suarez, who agreed with St Thomas on this, observed that while some precepts of the natural law are within the capacity of all to grasp, such as those prohibiting adultery and theft, others are not within the immediate capacity of all to grasp.[3] And the examples he gave of this latter category are that fornication is intrinsically evil, that usury is unjust and that lying can never be justified.

Given how tempting it is to tell a lie on many occasions – and how easily we give in to temptation – St Thomas’s absolute condemnation of lying is uncomfortable, even though he would be the first to point out that the remedy is available: repentance.[4] The question, though, is not whether St Thomas is uncomfortable, it is whether he is right. And that is, and always has been, a challenging question.

A controversial proposition

St Thomas looks back to the trenchant condemnation of lying in Aristotle’s Ethics[5] and to St Augustine of Hippo.[6] Later, equally firm condemnations of lying per se can be found in the Catechism of the Council of Trent, in Pascal, in numerous protestant theological works and famously in Kant.[7]

The proposition that the prohibition on lying is exceptionless has been challenged by a number of reputable theologians and philosophers, all of whom have been completely clear that while in general lying is wrong, there are occasions when it is permissible. In his Apologia Pro Vita Sua,[8] St John Henry Newman discussed in some detail the position of, among others, St Alphonsus Liguori and Clement of Alexandria, both of whom held that there were ‘special’ or ‘extraordinary’ circumstances that would justify telling a lie – in the words of St Alphonsus, that there was a justa causa. An example of such a justa causa is offered by Alasdair Macintyre:[9] a Dutch woman in 1942 took into her house a Jewish child and was asked by the Gestapo if all the children in her house were her own. She replied, ‘yes’. If she had said ‘no’, the child would have been seized and sent to Auschwitz.

Such an example gives us pause for thought. Newman summarises St Alphonsus’s reasoning on the matter as follows in the Apologia:

St. Alfonso, in another treatise, quotes St. Thomas to the effect, that, if from one cause two immediate effects follow, and, if the good effect of that cause is equal in value to the bad effect (bonus æquivalet malo), then nothing hinders that the good may be intended and the evil permitted. 

St Thomas’s position

St Thomas displays three distinct lines of thought in the two articles addressing the virtue of truthfulness and the vice of lying.

The first – which has been set out – explains that any lie is an offence against truth. Truthfulness as a virtue makes our choices right so lying necessarily misdirects us from the path to eternal salvation. This argument can be reinforced by arguments from some modern philosophers of language, who have proposed that there is a semantic rule requiring truth-telling in speech-acts of assertion, and that language cannot work without this rule.[10] Indeed, liars exploit this feature of language, for their lies would not benefit them if those to whom they lied did not regard their utterances as truthful.

The second is St Thomas’s commentary on the relationship between truth and justice: ‘Since humans are social animals, one human naturally owes another whatever is necessary for the preservation of human society. Now it would be impossible for humans to live together, unless they believed one another, as declaring the truth one to another.’[11] Although there are in a fallen world many cases of dishonesty, the institutions on which we depend both economically and politically only function because a degree of trust is maintained.

The third concerns the gravity of individual lies. St Thomas draws a distinction between those lies which do nobody any serious harm and those that do. The former, argues St Thomas, constitute venial sins, the latter grave or mortal sins.[12] St Thomas says broadly that lies either made in jest or to help others do not cause harm. However, he adds a caveat that those who have a duty to proclaim truth could sin mortally by breaching this duty.[13] Such a duty will usually be set out in some binding law, which as Francisco Suarez argued in detail in De Lege includes ‘mos’ – custom or convention. Deliberately to make or fail to correct promptly a false statement to the House of Commons is an example of a breach of the duty St Thomas has in mind.

Not all untrue statements are lies

It is possible to make a mistake. One may, for instance, be misinformed or have misunderstood something said or read. It may be embarrassing to correct an error, but it is not a sin to make a mistake unless one has failed to exercise the care and skill for which a situation obviously calls. There is often an obligation to correct a mistaken statement, and those who do not do so may well lose the protection that the honesty of the original error conferred. An example of this is the convention that a Member of Parliament who makes an erroneous statement to the House of Commons corrects it at the earliest opportunity.

St Thomas defines truth as adaequatio rei et intellectus[14] – ‘the equation of thought and thing’. Over time, our understanding of the world changes. Progress in science is made when one account of the world is replaced by another. For example, Aristotle’s description of physics was displaced by Newton’s, whose own ideas have since been superseded, but nobody would claim that Aristotle or Newton were lying in the accounts that each provided of the laws of motion.

Problems in St Thomas’s position

The Summa Theologica appears to be inconsistent on untruths in drama and more generally in works of fiction. The total condemnation of lies in Question 110 (which is part of the discussion of the cardinal virtue of justice and virtues annexed to it) would appear to embrace these. However, in Question 168 (which is part of the discussion of the cardinal virtue of temperance), St Thomas is emphatic that play (expressly including drama) is necessary for human flourishing.[15] This from a man who had himself been the target of vigorous satire in 1253 during a dispute in Paris. While St Thomas quotes Cicero and St Ambrose to the effect that there are limits to what is permissible on stage, for instance to prevent obscenity,[16] in this discussion he at no point suggests that untrue statements (or distortions of reality) in drama or satire are impermissible.

This raises a question over St Thomas’s earlier insistence that lies told in fun are sins. He expressly asserts that although the speaker is not seeking to deceive and that nobody is deceived, nevertheless the very nature of the action is to deceive.[17] Treating drama as not just permissible but a praiseworthy activity seems at odds with the earlier argument that because truthfulness is a virtue, lying is an offence against truth and a sin. There is a prima facie inconsistency here.

The case of the Dutch woman shielding a Jewish child raises a different issue. Unlike the actor in or author of a play that the audience knows contains untruths so does not actually deceive, she had the intention of (and one hopes she succeeded in) deceiving the man from the Gestapo. However, while all the ingredients for lying are present – the statement she made was untrue, she knew it to be, and she deliberately made it to deceive – we are very reluctant to regard what she did as a sin. If the lie had been detected she would herself have been shot or sent to Auschwitz. Surely to risk one’s life to save a child is not a sin but an act of heroic virtue.

The actual words of St Thomas suggest that something has gone wrong. When he discusses St Augustine’s division of sins into eight categories, in writing of some – including the seventh, a lie which saves someone from death – he acknowledges explicitly that the intentum is good.[18]

How did St Thomas get into this apparently paradoxical position? First, he seems like many modern philosophers to have regarded truth and telling the truth as a necessary condition for human flourishing (or even being able to communicate), and that deliberately or even carelessly to tell an untruth is to tell a lie and that is an offence. We can see how he is right: lies in commerce and politics are certainly damaging. The arguments that such activities as insider trading do not constitute ‘victimless crimes’ are powerful and suggest that when St Augustine writes of a lie ‘which injures no one, and profits someone in saving his money’ he was not familiar with some uncomfortable facts about commerce.

St Thomas also takes seriously the condemnations of lying found in scripture. He derives the separation of lies into those told to protect another, those told as jokes and those told with malign intent from a gloss on Psalm 5:6: ‘there are three kinds of lies; for some are told for the wellbeing and convenience of someone; and there is another kind of lie that is told in fun; but the third kind of lie is told out of malice.’[19] However, the author of the gloss – and so St Thomas – may have misread the psalm which actually says: ‘You will destroy all those who speak falsehood: the Lord abominates the man of blood and the treacherous.’[20] St Thomas only quotes the first half of the verse and the whole verse could be read as targeting only malevolent lies which do grave damage.

So, where do we stand?

A frequent position of those who hold that exceptions can be made to the precept commanding truthfulness is to adopt a rule that runs: ‘Always tell the truth, except…’ This is a dangerous approach, because it opens the way to endless exceptions, many of which are self-serving. We would rapidly find ourselves caught in that downward spiral of one mortal sin leading to another, as St Ignatius described desolation.[21] How might we proceed?

We should – as St Thomas does in the discussion of temperance – recognise that dramatic presentations and works of fiction do not aim to deceive and, in general, do not deceive. This is not to say that there should be no limits, but that the virtue of truthfulness in fiction does not generate obligations in the way it does in, for example, the House of Commons.

If we accept that telling the truth is a good, but recognise that it is not the only good and may on occasion be in conflict with other goods – and as we have seen, St Alphonsus thinks that this is actually St Thomas’s position – then the way is open to using the ways of making a choice between goods recommended by St Ignatius: to imagine oneself on one’s death bed,[22] to imagine oneself at the Day of Judgment[23] and carefully to set out the arguments for and against doing something.[24] None of these approaches is likely to allow self-serving lies.

These are approaches that respond positively to the writings of St Thomas. As he himself reminds us, the Summa Theologica is not Holy Scripture.[25] Indeed, the Summa is radically incomplete, in that St Thomas’s way of approaching a question was to set out the strongest arguments he could find against a proposition he was to defend and produce compelling responses to those arguments. The incompleteness of the Summa lies in the possibility of a new argument emerging that requires acceptance of a position rejected in our text. A robotic application of conclusions written down in the text is a betrayal of St Thomas. We respect him by engaging constructively with his thought.


Joe Egerton has written regularly on St Thomas Aquinas for Thinking Faith. His interest in St Thomas goes back to his time at Stonyhurst. He has been a NATO Research Fellow at Oxford, and subsequently worked in regulatory compliance. He stood as Conservative candidate for the Leigh constituency in 1992 and has more recently stood as an independent in Kent.


[1] The text of the Summa Theologica (ST) and all other works by St Thomas are available online at http://www.corpusthomisticum.org/ ST IIa IIae Q110.

[2] ST IIa IIae Q110 Art 3 co.

[3] Francisco Suarez, On Law and God the Lawgiver (‘De Lege’) [English translation in Thomas Pink (ed.), Selections from Three Works (Oxford, 1966) p. 235].

[4] See St Thomas’s commentary on the Miserere Psalm 51 (50).

[5] Aristotle, Nicomachean Ethics, 1127b4-8.

[6] St Augustine, De Mendacio.

[7] Kant’s conclusion may have agreed with St Thomas’s but he argued from very different premisses from those of St Thomas, and his account of perception, which followed Hume’s, excluded the possibility of St Thomas’s definition of ‘truth’.

[8] The Apologia is available online as a Project Gutenberg ebook. www.gutenberg.org/files/19690/19690-h/19690-h.htm The relevant discussion is in Section 8 of the Appendix ‘Answer in Detail to Mr Kingsley’s Accusations’

[9] Alasdair Macintyre, ‘Truthfulness and Lies: What can we learn from Mill?’ In Ethics and Politics, Selected Essays: Volume 2 (Cambridge 2006).

[10] See, for example: David Lewis, ‘Languages and Language’ in Philosophical Papers (Oxford, 1983); Mary Catherine Gormally, ‘The Ethical Root of Language’ in Peter Greach (ed.), Logic and Ethics (Dordrecht, 1991).

[11] ST IIa IIae Q109 Art3 ad 1.

[12] ST IIa IIae Q 110 Art 4.

[13] ST IIa IIae Q 110 Art 4 Ad 5

[14] ST Ia Q16 A1 co; see also ‘conformia rebus’ at ST IIa IIae Q109 a1 ad 3.

[15] ST IIa IIAe Q 168 a 2, summarised at ST IIa IIae A3 ad 3.

[16] ST IIa IIae Q168 a 2 co and ad1.

[17] ST IIa IIae Q110 Art 3 ad 6.

[18] ST IIa IIae Q110 Art 2 co.

[19] ST IIa IIae Q110 Art 2 s.c.

[20] Nicholas King’s translation (Kevin Mayhew, 2004), p. 1049.

[21] Ignatius of Loyola, Spiritual Exercises §314

[22] Spiritual Exercises §186

[23] Spiritual Exercises §187

[24] Spiritual Exercises §178

[25] ST Ia Q1 Art 8 ad 2.


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