Theorem exgen 1978

Description: Rule of existential generalization, similar to universal generalization ax-gen 1796, but valid only if an individual exists. Its proof requires ax-6 1970 in our axiomatization but the equality predicate does not occur in its statement. Some fundamental theorems of predicate calculus can be proven from ax-gen 1796, ax-4 1810 and this theorem alone, not requiring ax-7 2015 or excessive distinct variable conditions. (Contributed by Wolf Lammen, 12-Nov-2017.) (Proof shortened by Wolf Lammen, 20-Oct-2023.)

Hypothesis

Ref	Expression
exgen.1	⊢ 𝜑

Assertion

Ref	Expression
exgen	⊢ ∃𝑥𝜑

Proof of Theorem exgen

Dummy variable 𝑦 is distinct from all other variables.

Step	Hyp	Ref	Expression
1		idd 24	. 2 ⊢ (𝑥 = 𝑦 → (𝜑 → 𝜑))
2		exgen.1	. 2 ⊢ 𝜑
3	1, 2	speiv 1976	1 ⊢ ∃𝑥𝜑

Syntax hints:  ∃wex 1780

This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8  ax-gen 1796  ax-4 1810  ax-6 1970

This theorem depends on definitions:  df-bi 209  df-ex 1781

This theorem is referenced by:  extru  1980  19.2  1981  ac6s6  35452