Theorem stdpc4

Description: The specialization axiom of standard predicate calculus. It states that if a statement ph holds for all x , then it also holds for the specific case of t (properly) substituted for x . Translated to traditional notation, it can be read: " A. x ph ( x ) -> ph ( t ) , provided that t is free for x in ph ( x ) ". Axiom 4 of Mendelson p. 69. See also spsbc and rspsbc . (Contributed by NM, 14-May-1993) Revise df-sb . (Revised by BJ, 22-Dec-2020)

	Ref	Expression
Assertion	stdpc4	$$⊢\forall x\phi \to \left[t/x\right]\phi$$

Proof

Step	Hyp	Ref	Expression
1		ala1	$$⊢\forall x\phi \to \forall x\left(x=y\to \phi \right)$$
2	1	a1d	$$⊢\forall x\phi \to \left(y=t\to \forall x\left(x=y\to \phi \right)\right)$$
3	2	alrimiv	$$⊢\forall x\phi \to \forall y\left(y=t\to \forall x\left(x=y\to \phi \right)\right)$$
4		dfsb	$$⊢\left[t/x\right]\phi \leftrightarrow \forall y\left(y=t\to \forall x\left(x=y\to \phi \right)\right)$$
5	3 4	sylibr	$$⊢\forall x\phi \to \left[t/x\right]\phi$$