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I was looking online to see if this argument was a fallacy, but I couldn't find anything. It reminds me of the problem of induction, whereby one cannot predict what the future holds based on past events of some one or more things happening.

I'm leaning toward to the belief that for someone to argue "Once a thief, always a thief" means that the person has engaged in a hasty generalization.

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  • I'm having trouble finding it but I think there's a better fallacy than hasty generalization or slipper slope, and that's (from another context) steady state assumption. But I don't know what to call it here.
    – Joshua
    Mar 31, 2023 at 19:19
  • It takes a thief to catch a thief.
    – Scott Rowe
    Apr 1, 2023 at 22:17

7 Answers 7

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What kind of fallacy it is will depend on the underlying reasoning behind the statement. To lay this out as a valid argument, we need to dispel the assumptions. I think the natural read of this is:

1. Person A has once been a thief.

2. If someone been a thief at time t1 (once a thief), then they will be a thief at all times tx (always a thief).

C: Therefore, Person A is always a thief.

Here, you're right, Premise 2 may be a hasty generalisation. You are generalising the "being a thief" trait from time t1 to all times. (I say "may be" because this generalisation is not as hasty as most hasty generalisations need to be for them to be fallacies.)


However, you can make a similar argument saying:

1. Person A stole once.

2. If Person A stole once, it means that he has no moral compass.

3. If Person A has no moral compass, then it is likely that he will steal again.

4. Therefore, Person A will steal again.

C: Person A is always a thief.

Here, the fallacy being made with Premise 2 and 3 is more akin to a slippery slope fallacy.


In some sense, the claim, "Once a thief, always a thief", isn't an argument so it can't be fallacious. It's when you elaborate on the underlying argument that you get a flavour of a fallacy.

There's also ways to elaborate on it that aren't fallacious even if they might still be debatable, but that's beyond the scope of this answer.

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  • 16
    and if you define a thief as "someone who stole something" it is not fallacious because once you steal something you always will have stolen it.
    – user253751
    Mar 31, 2023 at 9:26
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    @user253751 It is still fallacious, because one is only a thief after one's first theft.
    – Peter
    Mar 31, 2023 at 11:52
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    @user253751 A person who stole something, and had some kind of change of character, wherein he apologised and returned the item or otherwise made good the damage ... I'd say he or she is no longer a thief.
    – Stewart
    Mar 31, 2023 at 13:59
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    @Stewart Not if you define "thief" as "someone who stole something", as they specified. :P
    – Idran
    Mar 31, 2023 at 14:11
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    @Stewart Yeah, but that's now working with a different definition, and so making a different point. You're right that your definition is more useful, but user253751 was specifically saying that under that particular definition, the statement isn't fallacious at all.
    – Idran
    Mar 31, 2023 at 15:02
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I depends on what you mean by "being a thief". For example if you define being a thief as "having stolen at least once", then it's not a fallacy but trivially true, if you've stolen once you're once and for all a thief forever so once a thief always a thief. However you could also define a reset on that property, for example by being caught/turn yourself in, punished and rehabilitated. In that case you'd be able to go from "thief"-> "no thief" and then it's not obvious why you should return to being a thief again, so the argument that you'll again be a thief because you were a thief is wrong. Same for when being a thief is merely about the act of stealing something, so then the argument would be "because you stole once, you'll always steal" which is in that generality not only not obviously true but kinda actually practically false as there will always be things that you won't steal because you can't or don't want to, so having stolen once doesn't mean you'll steal everything.

So the truth value of that statement various significantly depending on what you mean by being a thief. And the question of definition is a significant one because it actually makes a big difference for the moral and legal system concerning crime, punishment, rehabilitation and so on.

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  • Becoming a reformed thief doesn't mean that you're not a thief
    – Valorum
    Mar 31, 2023 at 18:01
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    Playing on ambiguity and different possible definitions is a well-known tactic of demagogy. I would argue that the fact that "being a thief" is ambiguous makes "Once a thief, always a thief" all the more a fallacy.
    – Stef
    Apr 1, 2023 at 12:49
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"Once a thief, always a thief" isn't an argument. It's a proposition, like "all men are mortal", which may be true or false.

"Socrates once stole something; once a thief, always a thief; therefore, Socrates is a lifelong thief" is a valid argument, given reasonable interpretations of the words in it. Whether you should accept the conclusion depends on whether you accept the premises, not on the structure of the argument.

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    I don't think it's necessarily just a proposition either (though it can be). After all, you could say that the argument is, "1. Once a thief; C: Always a thief". It's not a very good or complete argument, and it's not logically sound without adding additional propositions to dispel the assumptions around it, but it's still an argument of some sort. It's not just a claim like "All men are mortal" Mar 31, 2023 at 8:37
  • I think the premise is "a dog doesn't change its spots".
    – Barmar
    Mar 31, 2023 at 13:53
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    Note that only formal fallacies relate to structurally invalid arguments. Informal fallacies apply to flawed premises too. So it doesn't really answer the question to point out that you can put the possibly-flawed premise into a valid argument.
    – NotThatGuy
    Mar 31, 2023 at 15:03
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    No, "Once a thief, always a thief" is not a proposition. It's a sentence, and an ambiguous one. It doesn't map to a proposition until you start to interpret it. It's not just "true or false". It's full of half-expressed assumptions and designed to lead to false conclusions.
    – Stef
    Apr 1, 2023 at 12:50
7

"Once a thief, always a thief" is a saying, not an argument

If we want to turn it into an argument, there are a few possible interpretations: (among others)

A "thief" is defined as someone who's stolen once

This is not a truth statement, but rather a definition.

We could argue about whether it's a useful definition, but definitions don't have any truth value, and therefore this cannot be true or false.

If someone has stolen once, they'll steal whatever they can, at every available opportunity

This is fairly absurd and trivially false, as demonstrated by all the people who've stolen things once and don't try to carry away everything that isn't nailed down wherever they go.

Stealing requires a specific set of (possibly undefined) internal traits, only people with these traits steal, and these traits are permanent

This is probably unfalsifiable, so it wouldn't be particularly reasonable to believe it.

If someone points out everyone who's stolen in the past, but now seems to have rehabilitated and haven't appeared to steal anything since, one might simply respond that they haven't stolen again yet, or that we know of. That would not be a rational belief.

In any case, what would lead someone to stealing is a psychological question that's a lot less trivial than what's presented here, and evidence indicates that rehabilitation is possible.

Stealing generally requires some amount of moral bankruptcy, and this suggest that one would be more likely to steal again in future, and you shouldn't trust such a person to not steal again

This interpretation treats the saying as hyperbole, and it's probably reasonable to believe to some degree.

This is a less extreme version of the previous interpretation, and there are spectrums of how you judge different acts of stealing, how likely someone is to steal again, how much you should trust this person at various points in time, and the degree to which this person can regain one's trust, and each of these judgements may be more or less reasonable.


How the saying is typically meant is probably roughly one of the last two.


the problem of induction, whereby one cannot predict what the future holds based on past events

Predicting what the future holds based on past events is required for pretty much every part of modern society and human life.

We can't conclusively and absolutely know that things will continue to behave consistently (we can conclusively and absolutely know pretty much nothing), but it is most reasonable to believe that they will (within appropriately demonstrable bounds).

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  • Quick note on the problem of induction: The problem, as laid out by Hume, is that the inductive method cannot be justified by reason. While you're right that induction is useful, that is not enough to solve that problem. In addition, it is very difficult to justify, "it is most reasonable to believe that they will" without making circular claims. Why is it "most reasonable"? It's not a problem that can be so easily dismissed. Mar 31, 2023 at 15:30
  • @TheThoughtDetective If we're at a point t and induction works up to t+n (i.e. events prior to t predict events between t and t+n), this would serve as evidence that induction works (including after t+n). We have plenty of such evidence. "Induction works" is a scientific claim more than a logical one, and as a scientific claim, it's a question of evidence for and against, and the simplicity of the claim. There is probably a lot more to be said on the subject, but this is probably sufficient to address it on a basic level.
    – NotThatGuy
    Mar 31, 2023 at 15:50
  • If I understand this correctly, if we state the argument formally then our claim would be: 1. Induction predicts events prior to t+n; 2. If induction works for events prior to t+n, then induction works after t+n; 3. Thus induction works after t+n. However, Premise 2 here is induction. Thus, we need to presuppose induction to generate a justification for induction. // As for saying it is a scientific claim, I think that's a potential way to sidestep the problem by basically saying that it doesn't need to be justified by reason at all. That doesn't, however, solve the problem. Mar 31, 2023 at 16:58
  • @TheThoughtDetective Sure, I would agree that we cannot conclude that induction is true through logic alone. The truth of induction is demonstrated, not proven.
    – NotThatGuy
    Mar 31, 2023 at 17:21
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    Could be a fallacy of equivocation, between a thief is a person who stole anything ever, and a thief is a habitual thief, a career criminal.
    – Mary
    Mar 31, 2023 at 22:00
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In one sense if someone has committed thievery, that makes them a thief. They satisfy the definition, so it's not a fallacy. It doesn't necessarily speak to the intent to continue of a confirmed thief.

In another sense that it can be a suggestion that once someone has stolen it's inevitable that they will so so again. But this ignores the question of whether they might yield to temptation or intentionally steal.

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tl;dr The saying itself isn't fallacious; this is, it can be apt to observe that a person who was characteristically more prone to a behavior than others would likely tend to be more prone to that behavior than others in the future.

Of course, misapplications of concepts can be fallacious. For example, someone who stole an item once wouldn't necessarily always attempt to steal things in any given scenario – that'd be an over-generalization


As a saying, it's a bit terse so that it can be referenced concisely.

A more verbose, detailed version of this saying might be something like:

If someone was once a "thief" – this is, if it would've been accurate to have described their character as being prone to thievery – then that someone, so long as they remain essentially the same person in the sense of having the same general personality, would tend to retain that characteristic of being prone to thievery.

Of course, the expression could be pretty silly if someone mistook it for being literal. For example, if a thief were imprisoned where there was nothing for them to steal, then, obviously, they wouldn't steal.

But, again, it's just a terse reference to the general concept of how people tend to retain personality traits over time, including propensities toward behaviors such as thievery.

Also worth noting that someone having a propensity toward a behavior, e.g. thievery, wouldn't necessarily express that behavior in every scenario. This is, even someone who would be predisposed toward thievery wouldn't necessarily be fully defined by that one characteristic.


Discussion: Crimes can prove someone doesn't have a red-line prohibition against a behavior.

Some folks can have red-lines for behaviors that they won't engage in or even consider engaging in. For example, many folks would never rape someone under any circumstance – that's a red-line that they won't cross, or even consider crossing.

When someone does something, e.g. steals, they prove that they were capable of doing that thing, and therefore prove that they didn't have a red-line prohibition against it.

Of course, someone who might rape/steal/whatever wouldn't necessarily engage in the same behavior every time – in fact, they might never do it again!

That said.. sometimes it's not an issue of knowing that someone will do the behavior again, but rather an issue of not being able to trust that they wouldn't.

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It's not a fallacy for our aim is to reach our peak. For that reason only, we can't doubt the goodness in the world. Talking normally, it seems like the phrase is true because maybe we've only met thin men in life who couldn't make it upwards in life. The phrase points at the fragility of men. Well, there have been people who have woken up for the good and never turned back. I think it's not a question to be answered but understood. Glad for the question!

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