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Axiom ax-4 1452
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 1390. Conditional forms of the converse are given by ax-12 1454, ax-16 1749, and ax-17 1471.

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 1712.

(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 1294 . 2 wff 𝑥𝜑
43, 1wi 4 1 wff (∀𝑥𝜑𝜑)
Colors of variables: wff set class
This axiom is referenced by:  sp  1453  ax-12  1454  hbequid  1458  spi  1481  hbim  1489  19.3h  1497  19.21h  1501  19.21bi  1502  hbimd  1517  19.21ht  1525  hbnt  1595  19.12  1607  19.38  1618  ax9o  1640  hbae  1660  equveli  1696  sb2  1704  drex1  1733  ax11b  1761  a16gb  1800  sb56  1820  sb6  1821  sbalyz  1930  hbsb4t  1944  moim  2019  mopick  2033
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