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In propositional logic, a tautology is defined as a statement that is true no matter the truth values assigned to its propositional variables. So, in propositional logic, All apples are red or there exists an apple that is not red is a tautology.

Now, Wikipedia says:

A tautology in first-order logic is a sentence that can be obtained by taking a tautology of propositional logic and uniformly replacing each propositional variable by a first-order formula (one formula per propositional variable).

So, I understand that in first-order logic, (∀x∈N x is even) ∨ ¬(∀x∈N x is even) is a tautology as it is obtained by replacing the propositional variable A in the tautology A ∨ ¬A with (∀x∈N x is even).

However, I haven’t been able to find a text that states whether open sentences can be tautologies. For example, the open sentence (x is even) ∨ ¬(x is even) is obtained by replacing the propositional variable B in the tautology B ∨ ¬B with x is even. B would be an open sentence, which doesn’t have a truth value, so how could B ∨ ¬B be true? Is (x is even) ∨ ¬(x is even) a tautology?

ryang
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Dr. J
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  • For the variable B, is x assumed to be any real number? – X Stanton Aug 08 '23 at 00:23
  • No. No universe of discourse is specified so we don’t know what x is. – Dr. J Aug 08 '23 at 00:26
  • Udually, tautology applies to prop logic, while the corresponding concept for predicate logic is universally valid. BUT we can extend the concept of taut to predicate logic considering instances of propositional tautologies, like your example above: $(x=x) \lor \lnot (x=x)$. – Mauro ALLEGRANZA Aug 08 '23 at 10:14

3 Answers3

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Short answer: yes.

Long answer:

Say that a formula is elementary if it is either an atomic formula or a formula of the form $\exists x \phi$ for some $\phi$. Examples of elementary formulas are $P(x)$, $\exists x P(x)$, $\exists x \neg P(x)$, $\exists x (\neg P(x) \vee P(x))$. On the other hand, $\neg P(x)$ and $\exists x P(x) \rightarrow Q(y)$ are not elementary. Note that every quantified sentence is an elementary formula. For the purposes of defining tautological implication, quantified sentences are treated as black boxes, with no internal structure.

(Note also that, given this definition, one could define an appropriate propositional language by just taking the elementary formulas to be the propositional variables of this language (Enderton takes this approach: cf. A Mathematical Introduction to Logic, pp. 114ff). In that case, $P(x)$, $\exists x P(x)$, $\exists x \neg P(x)$, etc., would be treated as distinct sentence letters or propositional variables. They would be the same as $P, Q, R$, just with a different shape. One could then define a formula to be a tautology if it is a tautology in the propositional language generated by taking the elementary formulas as the propositional variables. It should be clear that this approach and the approach below are basically the same.)

Define a valuation $v$ as a function from the set of all elementary formulas to $\{0, 1\}$. This valuation can easily be extended to Boolean combinations of such formulas, by taking $v(\neg \phi) = 0$ iff $v(\phi)=1)$, etc. We can then say that a first-order formula $\phi$ is a tautology if $v(\phi)=1$ for every such valuation. (These definitions were taken from Shoenfield, Mathematical Logic, p. 26, but they are pretty standard.)

The idea is that such a formula is true by its mere propositional form, no matter its first-order content. If the formula is an open formula, then the idea is that its every instance will be true merely in virtue of its propositional form, where an instance of $\phi$ is the result of (uniformly) replacing its free variables for closed terms.

Notice that your example, $\forall x E(x) \vee \forall x \neg E(x)$ is not a tautology by this definition, since, in this approach, the negation sign inside the scope of the quantifier is black boxed. (In fact, since we took the existential quantifier as primitive, this sentence would actually be an abbreviation of $\neg \exists x \neg E(x) \vee \neg \exists x \neg \neg E(x)$, but the point still stands.) So this would be treated as similar to $P \vee Q$.

Finally, this notion of tautology is rather useful, since it allows us to use resources from propositional logic when dealing with first-order logic. In particular, since tautological consequence is decidable, this may be useful in searching for proofs---if the implication between two first-order formulas reduces to a tautological implication, there is actually an algorithm to find the proof of one from the other. This is one of the ideas behind Herbrand's Theorem.

Nagase
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  • Could you clarify what you mean by “every instance” in your last sentence. I think I understand but I just want to make sure. – – Dr. J Aug 08 '23 at 02:09
  • @Dr.J an instance of an open formula is the result of replacing its free variables with closed terms. – Nagase Aug 08 '23 at 10:30
  • so no matter what universe of discourse is chosen or where we place the quantifiers it is still a tautology? What about if the quantifiers were like this though: ($\forall$x)(x is even) $\lor$ ($\forall$x)(x is not even). From my understanding this isn’t a tautology in FOL – Dr. J Aug 08 '23 at 18:10
  • @Dr.J I've expanded my post to include examples, etc. I hope this is more informative, – Nagase Aug 08 '23 at 21:18
  • Ok so I think I get the idea. The only part that is still tripping me up is “ If the formula is an open formula, then the idea is that its every instance will be true merely in virtue of its propositional form”. When you say “every instance” are you meaning that it doesn’t matter what x is, (x is even or x is not even) will be true because of its propositional form? So if we plugged 2 in for x (x is even or x is not even) would be true. If we plugged in German Shepard for x (x is even or x is not even) would also be true. – Dr. J Aug 08 '23 at 23:14
  • So this has nothing to do with quantifiers it’s just saying that it doesn’t matter what value x is. Therefore since it doesn’t matter what value x is, it doesn’t matter if the (x is even or x is not even) is an open sentence, we still consider it a tautology, and it is also true. Am I understanding this correctly? – Dr. J Aug 08 '23 at 23:14
  • @Dr.J Yes, I think you understood it correctly. If the terminology of truth bothers you with regards to open formulas, you can use the term "valid" (indeed, Shoenfield defines validity with respect to a structure in this way; cf. p. 19). Notice that it is expected that "this has nothing to do with quantifiers", since tautology is a propositional notion. – Nagase Aug 08 '23 at 23:24
  • Ok I see. So (x is even or x is not even) is a tautology because the open sentence “x is even” can be assigned to the propositional variable A. Thus we get A v ~A which is a tautology. And (x is even or x is not even) is true in every instance. Therefore it is also valid. But it is also true because it is a tautology? – Dr. J Aug 08 '23 at 23:36
  • @Dr.J - I don't see any problem with calling it "true". – Nagase Aug 08 '23 at 23:38
  • Ok sounds good! Would you recommend reading the book you quoted from? Is it beginner friendly? Thank you – Dr. J Aug 08 '23 at 23:40
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No, if you take the stance that open sentences can't have a truth value, but there is a more fundamental misunderstanding here.

The variables we have in propositional and in first-order logic are of a different kind.

In propositional logic, we have variables for propositions, i.e. expressions that directly evaluate to true or false according to the valuation function. There is no notion of free or bound propositional variables, as there are no variable binding operators (no quantifiers or the like) in propositional logic.

In first-order logic, variables stand for individuals, and these individual variables are subject to binding by quantifiers ($\forall, \exists$), hence here we do have a distinction between free and bound variables, and thus closed and open sentences. In first-order logic, the expressions which have a truth value are complex formulas made up of (possibly) individual variables and predicates, and we speak of open or closed formulas depending on whether there occur any free variables in it.


Furthermore, "All apples are red or there exists an apple that is not red" is not a tautology in propositional logic. It is of the form $A \lor B$, not $A \lor \neg A$, because the second sentence is not a literal negation of the first, it is a syntactically different sentence.

"All apples are red or not all apples are red" would be of the form $A \lor \neg A$, and thus a tautology in propositional logic. Therefore $\forall x (\text{Apple}(x) \to \text{Red}(x)) \lor \neg \forall x (\text{Apple}(x) \to \text{Red}(x))$ (inserting $\forall x (\text{Apple}(x) \to \text{Red}(x))$ for $A$) must also be a tautology in first-order logic.

"There exists an apple that is not red" is logically equivalent to "Not all apples are red", but this can only be shown in first-order logic, because propositional logic is not fine-grained enough to express the quantifier structure. We can show that $\forall x (\text{Apple}(x) \to \text{Red}(x)) \lor \exists x (\text{Apple}(x) \lor \neg \text{Red}(x))$ is a tautology in first-order logic, but it is not an instance of any propositional logic tautology.

"$x$ is even" is not a formula in the language of propositional logic, we only have propositional variables available there. The best we can do in propositional logic is express this as $A \lor \neg A$ with propositional variable $A$, where the question of free or bound does not arise. In first-order logic, we can express it more fine-grained as $\text{Even}(x) \lor \neg \text{Even}(x)$, with the individual variable $x$ free, and this is indeed an open sentence which, depending on your semantics, can not be assigned a truth value. You could say that only first-order sentences obtained by substituting closed sentences for propositional variables are also tautologies because only they are guaranteed to have a truth value.

  • I see this makes a lot of sense. Thank you for the thorough explanation. Why would ∀x(Apple(x)→Red(x))∨∃x(Apple(x)∨¬Red(x)) not be a tautology in propositional logic? – Dr. J Aug 08 '23 at 02:56
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    First of all it is not a formula of propositional logic at all, but in its structure it is not something of the form $A \lor \neg A$, as the second sentence is not a literal negation of the first. That would have to be "Not all apples are red", but we only have "Some apples are not red", and these are not the literally same sentence. – Natalie Clarius Aug 08 '23 at 03:05
  • We do have that "All apples are red or not all apples are red" is a propositional tautology because that is of the form $A \lor \neg A$. But "All apples are red or some apples are not red" is a different statement, which we can only translate as $A \lor B$ so we can not show that that is tautological by the means of propositional logic. We need first-order logic to look into the sentence in more detail and explain why "Not all apples are red" means the same as "Some apples are not red"; propositional logic can not do that. – Natalie Clarius Aug 08 '23 at 03:11
  • Ok so I think I get it. So in propositional logic the negation of “All apples are red” is “not all apples are red”. In propositional logic it wouldn’t be “There exists an apple that is red” because propositional logic does not have quantifiers. In FOL the negation of “All apples are red” would be “there exits an apple that is not red”. So “all apples are red or there exists an apple that is not red” is not a tautology in propositional logic but it is a tautology in FOL. It is a tautology in FOL because the negation of “All apples are red” is “there exists an apple that is not red. – Dr. J Aug 08 '23 at 03:46
  • It is not a tautology in propositional logic because “there exists an apple that is not red” is not the negation of “All apples are red” in propositional logic. Am I understanding all this correctly? – Dr. J Aug 08 '23 at 03:47
  • Yes, that's correct. – Natalie Clarius Aug 08 '23 at 03:59
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Wikipedia says:

A tautology in first-order logic is a sentence that can be obtained by taking a tautology of propositional logic and uniformly replacing each propositional variable by a first-order formula (one formula per propositional variable).

Wikipedia is defining a tautology as a formula whose truth-functional form is true regardless of its atomic propositions' truth values; so, it does not admit x=x (whose truth-functional form is just P) as a tautology. On the other hand, some authors refer to every logical validity as a tautology, so call x=x a tautology.

Everyone, though, agrees that (x is even) ∨ ¬(x is even) (whose truth-functional form is P ∨ ¬P) is a tautology. Therefore, the answer to your main question

Can tautologies have free variables?

is Yes.

the open sentence (x is even) ∨ ¬(x is even) is obtained from B ∨ ¬B. B would be an open sentence, which doesn’t have a truth value, so how could B ∨ ¬B be true?

(x is even) ∨ ¬(x is even) is an open formula, but its truth-functional form B ∨ ¬B is a closed formula (i.e., a sentence) that is tautological.

In propositional logic, a tautology is defined as a statement that is true no matter the truth values assigned to its propositional variables. So, in propositional logic, All apples are red or there exists an apple that is not red is a tautology.

The first-order validity All apples are red or there exists an apple that is not red has truth-functional form P ∨ Q, so is actually not a propositional-logic tautology.

ryang
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  • Isn’t $x=y \lor \neg x=y$ just an instance of a propositional tautology? I don’t see why its truth-functional form is just $p$ as opposed to $p \lor \neg p$. – PW_246 Sep 05 '23 at 19:57
  • @PW_246 Thanks for catching that nonsense! I've revised the answer. – ryang Sep 06 '23 at 02:26