I have a question in my exam practice, to determine if the following statement is a tautology, in First Order Logic:


I think it is a tautology, but am I correct?

In my course the proffesor told us to "translate" the statement to Propositional Logic, like that:


Your help is appriciated, is the statment above a tautology in first order logic?


Yes, the first-order formula $\forall x (p(x) \rightarrow p(x))$ is valid and we can obtain it "generalizing" an instance of the tautology : $\varphi \rightarrow \varphi$.

I.e. we can obtain it from the tautology $\varphi \rightarrow \varphi$ replacing $\varphi$ with the formula $p(x)$.

Regarding $\forall x p(x) \lor \lnot \forall y p(y)$, it is obtained from $\varphi \lor \lnot \psi$, which is not a tautology.

But it is also a valid formula of first-order logic; in fact, it is equivalent to $\forall x p(x) \lor \lnot \forall x p(x)$ which is obtained from the tautology $\varphi \lor \lnot \varphi$.

  • $\begingroup$ Thank you, that's what I thought, I don't know why the answer the proffesor gave me is not the same. $\endgroup$ – Alan Feb 10 '15 at 21:30
  • $\begingroup$ But according to 'A Mathematical Introduction to Logic' by Enderton 2nd edition page 115, there is this sentence 'On the other hand, neither $\forall x (Px \rightarrow Px)$ nor $\forall x Px \rightarrow Px$' is a tautology'. $\endgroup$ – Idonknow Apr 12 '15 at 8:27

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