Axiom ax-c5 39698

Description: Axiom of Specialization. A universally quantified wff implies the wff without the universal quantifier (i.e., an instance, or special case, of the generalized wff). In other words, if something is true for all 𝑥, then it is true for any specific 𝑥 (that would typically occur as a free variable in the wff substituted for 𝜑). (A free variable is one that does not occur in the scope of a quantifier: 𝑥 and 𝑦 are both free in 𝑥 = 𝑦, but only 𝑥 is free in ∀𝑦𝑥 = 𝑦.) Axiom scheme C5' in [Megill] p. 448 (p. 16 of the preprint). Also appears as Axiom B5 of [Tarski] p. 67 (under his system S2, defined in the last paragraph on p. 77).

Note that the converse of this axiom does not hold in general, but a weaker inference form of the converse holds and is expressed as rule ax-gen 1828. Conditional forms of the converse are given by ax-13 2407, ax-c14 39706, ax-c16 39707, and ax-5 1943.

Unlike the more general textbook Axiom of Specialization, we cannot choose a variable different from 𝑥 for the special case. In our axiomatization, that requires the assistance of equality axioms, and we deal with it later after we introduce the definition of proper substitution (see stdpc4 2105).

An interesting alternate axiomatization uses axc5c711 39733 and ax-c4 39699 in place of ax-c5 39698, ax-4 1842, ax-10 2179, and ax-11 2195.

This axiom is obsolete and should no longer be used. It is proved above as Theorem sp 2222. (Contributed by NM, 3-Jan-1993.) Use sp 2222 instead. (New usage is discouraged.)

Assertion

Ref	Expression
ax-c5	⊢ (∀𝑥𝜑 → 𝜑)

Detailed syntax breakdown of Axiom ax-c5

Step	Hyp	Ref	Expression
1		wph	. . 3 wff 𝜑
2		vx	. . 3 setvar 𝑥
3	1, 2	wal 1568	. 2 wff ∀𝑥𝜑
4	3, 1	wi 4	1 wff (∀𝑥𝜑 → 𝜑)

This axiom is used by:  ax4fromc4  39709  ax10fromc7  39710  hba1-o  39712  equid1  39714  hbae-o  39718  ax12fromc15  39720  ax13fromc9  39721  sps-o  39723  axc5c7  39726  axc711toc7  39731  axc5c711  39733  ax12indalem  39760  ax12inda2ALT  39761