Some Metatheorems of Deduction

Mort Yao

Generalization Theorem. If \(\Gamma \vdash \varphi\) and \(x\) does not occur free in any formula in \(\Gamma\), then \(\Gamma \vdash \forall x \varphi\).

Proof. (By induction on \(\varphi\).)

Rule T. If \(\Gamma \vdash \alpha_1, \dots, \Gamma \vdash \alpha_n\) and \(\{\alpha_1, \dots, \alpha_n\} \models_t \beta\), then \(\Gamma \vdash \beta\).

Deduction Theorem. \(\Gamma; \gamma \vdash \varphi \iff \Gamma \vdash (\gamma \to \varphi)\).

Proof. (By induction on \(\varphi\).)

Contraposition. \(\Gamma; \varphi \vdash \lnot \psi \iff \Gamma; \psi \vdash \lnot \varphi\).

If for some \(\beta\), both \(\beta\) and \(\lnot \beta\) are theorems of the set \(\Gamma\), we say that \(\Gamma\) is inconsistent. In this case, any arbitrary formula \(\alpha\) is a theorem since \(\beta \to \lnot \beta \to \alpha\) is a tautology.

Reductio ad Absurdum. If \(\Gamma; \varphi\) is inconsistent, then \(\Gamma \vdash \lnot \varphi\).

Generalization on Constants. Assume that \(\Gamma \vdash \varphi\) and that \(c\) is a constant symbol that does not occur in \(\Gamma\). Then there is a variable \(y\) (which does not occur on \(\varphi\)) such that \(\Gamma \vdash \forall y \varphi^c_y\). Furthermore, there is a deduction of \(\forall y \varphi^c_y\) from \(\Gamma\) in which \(c\) does not occur.

Corollary 1. Assume that \(\Gamma \vdash \varphi^x_c\), where the constant symbol \(c\) does not occur in \(\Gamma\) or in \(\varphi\). Then \(\Gamma \vdash \forall x \varphi\), and there is deduction of \(\forall x \varphi\) from \(\Gamma\) in which \(c\) does not occur.

Existence of Alphabetic Variants. Let \(\varphi\) be a formula, \(t\) be a term, and \(x\) be a variable. Then we can find a formula \(\varphi'\) (called an alphabetic variant of \(\varphi\)), which differs from \(\varphi\) only in the choice of quantified variables such that

  1. \(\varphi \vdash \varphi'\) and \(\varphi' \vdash \varphi\).
  2. \(t\) is substitutable for \(x\) in \(\varphi'\).