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Axiom ax-4 1487
Description: Axiom of Specialization. A quantified wff implies the wff without a quantifier (i.e. an instance, or special case, of the generalized wff). In other words if something is true for all 𝑥, 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 1425. Conditional forms of the converse are given by ax-12 1489, ax-16 1786, and ax-17 1506.

Unlike the more general textbook Axiom of Specialization, we cannot choose a variable different from 𝑥 for the special case. For use, that requires the assistance of equality axioms, and we deal with it later after we introduce the definition of proper substitution - see stdpc4 1748.

(Contributed by NM, 5-Aug-1993.)

Ref Expression
ax-4 (∀𝑥𝜑𝜑)

Detailed syntax breakdown of Axiom ax-4
StepHypRef Expression
1 wph . . 3 wff 𝜑
2 vx . . 3 setvar 𝑥
31, 2wal 1329 . 2 wff 𝑥𝜑
43, 1wi 4 1 wff (∀𝑥𝜑𝜑)
Colors of variables: wff set class
This axiom is referenced by:  sp  1488  ax-12  1489  hbequid  1493  spi  1516  hbim  1524  19.3h  1532  19.21h  1536  19.21bi  1537  hbimd  1552  19.21ht  1560  hbnt  1631  19.12  1643  19.38  1654  ax9o  1676  hbae  1696  equveli  1732  sb2  1740  drex1  1770  ax11b  1798  a16gb  1837  sb56  1857  sb6  1858  sbalyz  1974  hbsb4t  1988  moim  2063  mopick  2077
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