The Liar's Paradox

I’ll have a go at explaining it, more so as to clear the argumet up for myself than to explain it.

There is a denumerable sequence of sentences in which each says that the conjunction of the rest of the sentences is false. So we have a sequence (S₁, S₂, S₃…) in which each sentence says that all the rest of the sentences, taken together, are false.

Each sentence is only about the subsequent sentences. So no sentence is about itself. No self reference.

Each sentence says that at least one of the subsequent sentences is false.

Suppose that Sₙ is true. Then at least one of the subsequent sentences is false. But each subsequent sentence says that at least one of the sentences subsequent to it is false, so if it is false, the subsequent sentences are true. But if the subsequent sentences are true, Sₙ is false.

So if Sₙ is true, then it is false.

Supose Sₙ is false. Then the conjunction of the rest of the sentences is true. But each of those says that some sentence subsequent to it is false. If some subsequent sentence is false, then the conjunction of those sentences is also false. But Sₙ says just that, and hence is true.

So if Sₙ is false, then it is true.

Something like that.

Did I kill this thread?

Oops.

:+1: Meaning exceeds classical logic.

killjoy.

You didn’t kill the thread.

The original text talked about Gödel diagonalization, new term for me as it usually has been referred to Gödel numbering (as in the incompleteness theorems)

So this (Yalbo’s paradox) is a bit similar, but naturally different, to the sentence A saying something about sentence B, which then refers to A. Yes, the self-reference isn’t in itself in any statement, the problem arises only when the thinking of all sentences together. And what is very clear is the “negative” part in the negative self-reference: This sentence is true, but at least one of the following sentences is false. No problem if this was positive self-reference: at least one of the following statements is true.

And I guess there’s an infinite amount of these, or in the case of there being then the last one refers to the first one? (In the finite case it’s more of the Sentence A → Sentence B → Sentence A)

Hopefully I got it, but correct me if you can.

So how do we go / transform something that was paradoxical to something that is an incompleteness result?

Here’s my pitch and please correct me if I’m wrong: In any paradox or antinomy we just assume that the premises are true. Starting with a premis like all mathematical statements are either false or true. Then with direct application of negative self-reference, we get a result “if true, then false” and “if false, then true”. Now we have gotten a paradox.

Yet the way Cantor, Turing and Gödel used the diagonal method (in a very generalized sense is negative self-reference) didn’t end up in a paradox, because the statements were reductio ad absurdum proofs: if we assume we can do something (from putting the reals into a 1-to-1 correspondce with the natural numbers or the Turing Machine can compute everything etc), they show cannot be done, hence there’s a limitation on what can be done. That isn’t a paradox.

Hopefully you can follow my thinking…

The Lair’s and other paradoxes.

Wim Dekkers, 5 April 2026

The question of logical paradoxes keeps many a philosopher busy. There are many different types of philosophical paradoxes, in this essay I will look into logical paradoxes, that seem to make no sense in the world of logical thinking, as their truth value seems to be impossible to establish. They are called antinomies, and are described as producing a self-contradiction by accepted ways of reasoning. In this category we have Bertrand Russell’s paradox about the barber who shaves all men in the village that do not shave themselves, and Russell’s famous paradox of the class of all classes that are not members of itself. Then there is the following logical paradox: is heterological heterological or autological (Grelling’s paradox) ? And a last famous example: all Cretans are liars, said by Epimenides the Cretan, the paradox of Epimenides, or the Liar’s paradox.

For philosophical logicians the antinomies paradoxes are deep logical contradictions, that no one understands. The question in this essay concerns the paradox of Epimenides specifically, a version of what is also called the Liar’s paradox. This paradox is especially confusing for logicians as the subject-matter is the concept of “truth”. Of course the “truth” is always involved in all the antinomies paradoxes, as logic defines these paradoxes as not being either true or false: it is impossible to define their truth value and that is why they are paradoxes in the first place. So, instead of being about men’s facial hair, mathematical classes, or adjectives, the Liar’s paradox is about the subject matter of “truth”, and as the other three paradoxes, the problem is that no truth value can be found. This is of course extra confusing, as the subject-matter and the reason for why it is a paradox both concern the “truth”, and now these two parts of the paradox, the subject-matter and the self-contradiction, are easily mingled in our human reasoning about it.

I will argue that all 4 of these paradoxes are simply erroneous reasoning, as a result of a wrong and a very confusing use of words. I will also argue that the problem about the Liar’s paradox has nothing more to do with the concept of “truth” as any of the other examples, and that the subject-matter, whether it is the “truth” or men’s facial hair or mathematical classes, has nothing to do with what the problem of confusion really creates. To do this I will analyse all four of our examples, to show the repetition of the same type of reasoning error made. The problem, that creates contradictions, lies in the meaning of the words, not in anything to do with formal logic or truth-value. I will start explaining all four mentioned logic paradoxes:

First, the barber shaves all men that do not shave themselves. Who shaves the barber ? If he does not shave himself, he belongs to the group of men that do not shave themselves, and so he has to shave himself, which is a contradiction. The problem is the definition of the barber, as someone who shaves the men of the island that do not shave themselves. This is a circular definition in the case that the barber is himself a man of the island that does not shave himself. Circular definitions are considered fallacious as they define a term in terms of themselves, and therefore do not define anything at all, but open an infinite circular reasoning. They are also begging the question. Now, on top of that, (and important for the creation of a contradiction), if you add a negative in the definition, this become immediately a contradiction. As in “this thing = not this thing”. In this case you would get something like “the man who is not shaving himself = not (the man who is not shaving himself)”. A man cannot at the same time and shave himself and not shave himself. So, the repetitive term in the circular definition, with an added negative, creates automatically a contradiction.

Next, is the adjective heterological heterological or autological? Heterological is the name for adjectives that do not describe themselves (like, German or monosyllabic), autological for adjectives that do describe themselves (like English or short). I think heterological and autological cannot be seen themselves as adjectives as they would then become circular definitions. They use already explicitly in their definition the concept of adjective, so you get the adjective is the adjective…which is circular. Added the negative of heterological and you get automatically a contradiction.

Then, the class of all classes that are not a member of itself. Is this class a member of itself? Again, as in the examples above: if you define a class with explicitly using itself in the definition, as happens already with classes that are defined as members of themselves, you have a circular definition (I can imagine that an infinite regress starts here in mathematics, but I am not sure, as I am not a mathematician). If you use the definition of classes that are not members of themselves, to define a new class, this definition of class is not only circular as it uses explicitly itself in it’s definition, but because you added the negative it herewith becomes a contradiction. This is how the paradox is born: a circular definition with an added negative in the definition.

Finally, all Cretans are liars, says the Cretan. Is the Cretan lying ? The circularity of the definition is clear and explicit: he, Epimenides, a talking Cretan that tells the truth, says all talking Cretans do not tell the truth. The problem is again the negative circularity in the definition. A talking Cretan (telling the truth, implicitly) defined as a talking Cretan not telling the truth. This is both a circular definition and, again, because of the negative in the definition (not telling the truth), it is also a contradiction.

So, here we have our paradoxes, which are all four circular definitions that do not define anything much but are fallacious as definitions and begging the question. But with an added negative in the definition they become contradictions. It is like defining a dog as being not a dog. Defining a dog as a dog is problematic, but defining a dog as not a dog is double problematic, and a contradiction. So that is the problem of the confusion of the 4 paradoxes, which is actually quite complicated erroneous use of language, that creates such confusion so that you do not see anymore what is actually the trouble: a double fallacy with the circular definition and a negative added that creates a contradiction.

So, we are here not arguing about self-reference, as is often done in discussions about these logical paradoxes, but about circular definition, which is a very special case of self-reference, with very special logical consequences. The added negative is finally responsible, in combination with the circular definition, to create the contradiction.

Now, if we go back to the second part of our question, whether one’s theory of truth makes a difference in whether the Liar’s paradox is a threat, I think it is now clear that that is not the case. That the Liar’s paradox has lying and telling the truth as it’s subject matter, just like the barber paradox has men’s care of facial hair as it’s subject matter, is not relevant for why it is a paradox. Like anyone’s ideas of men’s care of facial hair does not make the barber’s paradox more or less of a paradox. The problem with the paradoxes is not in the subject-matter but in the use of circular definition with a negative added to it. The fact that the subject-matter in the liar’s paradox is the concept of “truth”, and that the general problem of logical paradoxes is the impossibility of defining a truth value, just confuses even more the logical thinking about it, but it does not create the paradox.

Truth is not a predicate. Truth is built into the structure of logic. One doesn’t say, “The sun is a star is true”, one simply states, “The sun is a star.”

Circular arguments are not fallacious. Just unhelpful. And circular definitions are common and unproblematic. And Yalbo’s paradox explicitly does not involve circularity.

Not a first-order predicate, yes. Attempts to eliminate it entirely have mixed results. Hence the three approaches mentioned above make use of “truth” as a predicate.

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I am not sure what your conclusion is here.

The sequence set up by Yalbo is not circular.

My intuition, unformalised, is that Yalbo sets up a sequence that is undecidable. More accurately, it has no global grounding, since it’s not that any one item in the sequence is undecided, but that the sequence as a whole cannot be grounded.

Revision Theory is helpful here, since it correctly classifies every sentence in the sequence as unstable and so paradoxical. But unlike most other paradoxes it cannot say why.

The beauty here is in the process of transcendence; no sooner is a boundary proposed than some wag undermines it. Hence the connection to my thread on aesthetics as going beyond supposed limitations.

That’s not quite what the description of Yalbo’s paradox in the linked article says. It speaks of every subsequent sentence in the sequence being wrong, not just their conjunction:

Sn: For all m>n, Sm is false.

I don’t think this works.

  1. Suppose Sn is true. This will be satisfied if there exists m>n such that Sm is false.
  2. If Sm is false then for every o>m, So is true. Note that this does not contradict Sn, since Sn is already satisfied. However,
  3. For every o>m, So contradicts (2). Every So says that there is at least one subsequent sentence that is false, but by (2) they all must be true.

If Sn is false then we start at step (2) above and get to the same conclusion even faster. So, your version works, after all (if I got it right!)

I do think that there is both a kind of indexicality and a kind of self-reference here, which become apparent when one unpacks the meaning of S in each sentence. But that’s not to say that indexicality and/or self-reference are solely responsible for the paradox.

Yes, indeed. In my explanation I tried a move from universal quantification to existential quantification - from “All sentences are not…” to “Some sentence is…”. It probably needs work, but the idea was pedagogic, to give an alternative view of the paradox. The aim was to show how any self-reference is not immediate, and that no indexical is involved. The objection that there remains some sort of “holistic” self-reference is yet to be properly formalised.

The liar sentence is a performative contradiction. A statement is true and the liar is a statement, but it declares itself false (re Jürgen Habermas, RIP)

Not all statements are true, and not all statements are truth-apt.

Meaning? Some sentences are neither true nor false? We could say that but to look at the other option - the structure of language and logic, how they interact - looks more fruitful.

That is the structure of language and logic.

I guess so. The liar is neither true, nor false. If not, it’s a contradiction, prompting some philosophers to adopt positions like dialetheism/into inventing systems like paraconsistent logic.

Yes. See my main post, above.

I thought that the liar’s sentence is not truth-apt at one point.

But the strengthened liar’s sentence persuaded me elsewise.

From the SEP on Dialethism:

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@Banno A link would’ve been nice. A short explanation nicer.

@Moliere Banno says the liar isn’t truth-apt. I say the issue is that the liar has truth as a predicate. I wanted to discuss how language and logic are related. A declarative sentence is meant as true/false and it doesn’t come with an end-note stating that it is so. For example, “the table is a mammal” is logic-apt, it is false, but “the table is a mammal, is false” is not. The latter is a meta-statement; it’s place in 1st order logic isn’t well defined.

Consider the statement, 2 + 2 = 4 and “2 + 2 = 4,, is a mathematical statement” We can continue doing math with one (4 + 4 = 8), but the other is 100\% resistant to any further math.

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As I alluded to earlier, the liar is a (performative) contradiction. To state a proposition is to also state that it is true. The liar when stated is as true, but it says of itself that it’s false. We could/should stop there, no? I find this interpretation more elegant than your, not all statements are …