Substitution Instance of WFF is WFF
Theorem
Let $\mathbf A$ be a WFF of predicate logic.
Let $\tau$ be a term of predicate logic.
Let $x \in \mathrm{VAR}$ be a variable.
Let $\mathbf A \left({x \gets \tau}\right)$ be the substitution instance of $\mathbf A$ substituting $\tau$ for $x$.
Then $\mathbf A \left({x \gets \tau}\right)$ is a WFF.
Proof
Proceed by the Principle of Structural Induction on the bottom-up specification of predicate logic, applied to $\mathbf A$.
If $\mathbf A = p \left({\tau_1, \ldots, \tau_n}\right)$, then:
- $\mathbf A \left({x \gets \tau}\right) = p \left({\tau_1 \left({x \gets \tau}\right), \ldots, \tau_n \left({x \gets \tau}\right)}\right)$
where $\tau_i \left({x \gets \tau}\right)$ is the substitution instance of $\tau_i$.
By Substitution Instance of Term is Term, each such $\tau_i \left({x \gets \tau}\right)$ is again a term.
It follows that $\mathbf A \left({x \gets \tau}\right)$ is again a WFF.
If $\mathbf A = \neg \mathbf B$ and the induction hypothesis applies to $\mathbf B$, then:
- $\mathbf A \left({x \gets \tau}\right) = \neg \mathbf B \left({x \gets \tau}\right)$
and it follows from the induction hypothesis that $\mathbf A \left({x \gets \tau}\right)$ is a WFF of predicate logic.
If $\mathbf A = \mathbf B \circ \mathbf B'$ for $\circ$ one of $\land, \lor, \implies, \iff$ and the induction hypothesis applies to $\mathbf B, \mathbf B'$:
- $\mathbf A \left({x \gets \tau}\right) = \mathbf B \left({x \gets \tau}\right) \circ \mathbf B' \left({x \gets \tau}\right)$
and it follows from the induction hypothesis that $\mathbf A \left({x \gets \tau}\right)$ is a WFF of predicate logic.
If $\mathbf A = \exists x: \mathbf B$, and the induction hypothesis applies to $\mathbf B$:
- $\mathbf A \left({x \gets \tau}\right) = \exists x: \mathbf B \left({x \gets \tau}\right)$
and it follows from the induction hypothesis that $\mathbf A \left({x \gets \tau}\right)$ is a WFF of predicate logic.
If $\mathbf A = \forall x : \mathbf B$, and the induction hypothesis applies to $\mathbf B$:
- $\mathbf A \left({x \gets \tau}\right) = \forall x: \mathbf B \left({x \gets \tau}\right)$
and it follows from the induction hypothesis that $\mathbf A \left({x \gets \tau}\right)$ is a WFF of predicate logic.
The result follows by the Principle of Structural Induction.
$\blacksquare$
Sources
- 2009: Kenneth Kunen: The Foundations of Mathematics ... (previous) ... (next): $\mathrm{II}.8$ Further Semantic Notions