The Thomistic Institute 'I Cannot Tell a Lie': Thomas Aquinas on the Moral Permissibility of Lying – Prof. Christopher Tomaszewski
May 6, 2026
Christopher Tomaszewski, Assistant Professor of Philosophy at Belmont Abbey College, explores Thomas Aquinas on lying. He defines lying as assertion contrary to belief and treats assertions as speech acts. He examines exceptions (plays, poker), scriptural and catechism views, natural law arguments that lying perverts the faculty of assertion, and thorny cases like Nazis at the door, white lies, and state deception.
AI Snips
Chapters
Transcript
Episode notes
Lying Perverts The Faculty Of Assertion
- Lying perverts the faculty of assertion by using speech to invite belief while simultaneously undermining that very purpose.
- Tomaszewski parallels this to ingesting rat poison: assertion's end is to report belief, so asserting contrary to belief subverts the faculty.
Irrationality Shows Why Perverted Acts Are Immoral
- Deliberately frustrating one's rational ends is intrinsically irrational and therefore immoral on a Thomistic natural law account.
- Tomaszewski argues perverted acts involve a performative contradiction, like building a house on sand, showing moral irrationality.
Prefer Withholding Or Evasive Truths Over Lying
- Distinguish truthfulness from candor and withhold truths when contextually appropriate instead of lying.
- Tomaszewski notes duties of candor exist (e.g., lawyers, doctors, minors to parents), but silence or truthful evasions are often permissible.