Axiom ax-c5 36824

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 1799. Conditional forms of the converse are given by ax-13 2372, ax-c14 36832, ax-c16 36833, and ax-5 1914.

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 2072).

An interesting alternate axiomatization uses axc5c711 36859 and ax-c4 36825 in place of ax-c5 36824, ax-4 1813, ax-10 2139, and ax-11 2156.

This axiom is obsolete and should no longer be used. It is proved above as Theorem sp 2178. (Contributed by NM, 3-Jan-1993.) Use sp 2178 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 1537	. 2 wff ∀𝑥𝜑
4	3, 1	wi 4	1 wff (∀𝑥𝜑 → 𝜑)

This axiom is referenced by:  ax4fromc4  36835  ax10fromc7  36836  hba1-o  36838  equid1  36840  hbae-o  36844  ax12fromc15  36846  ax13fromc9  36847  sps-o  36849  axc5c7  36852  axc711toc7  36857  axc5c711  36859  ax12indalem  36886  ax12inda2ALT  36887