Definition df-rals 50854

Description: Define "all some" applied to a class, which means 𝜓 is true whenever 𝜑 is true for 𝑥 in 𝐴, and there is at least one 𝑥 in 𝐴 where 𝜑 is true.

An older definition of the "all some" quantifier when scoped to a class, named df-alsc and now removed, instead applied a bare formula 𝜑 to the members of a class, asserting only (∀𝑥 ∈ 𝐴𝜑 ∧ ∃𝑥𝑥 ∈ 𝐴), that is, that the formula held throughout 𝐴 and that 𝐴 had at least one member. I've now decided that that was a mistake. Its older existence conjunct ∃𝑥𝑥 ∈ 𝐴 did not require any member of 𝐴 to satisfy the antecedent, so if the formula was itself an implication, that inner implication could still be vacuously true, which is precisely what the allsome quantifier exists to prevent. For example, the older definition meant that "among Martians, all tall ones are green" could be considered true if there are Martians, but no tall Martians. This version of the definition instead ensures that claims of the form "among Martians, all tall ones are green" can only be true if all tall Martians are green and that there is at least one tall Martian. (Contributed by David A. Wheeler, 20-Oct-2018.) (Revised by David A. Wheeler, 12-Jul-2026.)

Assertion

Ref	Expression
df-rals	⊢ (∀∃𝑥 ∈ 𝐴(𝜑 → 𝜓) ↔ (∀𝑥 ∈ 𝐴 (𝜑 → 𝜓) ∧ ∃𝑥 ∈ 𝐴 𝜑))

Detailed syntax breakdown of Definition df-rals

Step	Hyp	Ref	Expression
1		wph	. . 3 wff 𝜑
2		wps	. . 3 wff 𝜓
3		vx	. . 3 setvar 𝑥
4		cA	. . 3 class 𝐴
5	1, 2, 3, 4	wrals 50852	. 2 wff ∀∃𝑥 ∈ 𝐴(𝜑 → 𝜓)
6	1, 2	wi 4	. . . 4 wff (𝜑 → 𝜓)
7	6, 3, 4	wral 3077	. . 3 wff ∀𝑥 ∈ 𝐴 (𝜑 → 𝜓)
8	1, 3, 4	wrex 3087	. . 3 wff ∃𝑥 ∈ 𝐴 𝜑
9	7, 8	wa 401	. 2 wff (∀𝑥 ∈ 𝐴 (𝜑 → 𝜓) ∧ ∃𝑥 ∈ 𝐴 𝜑)
10	5, 9	wb 209	1 wff (∀∃𝑥 ∈ 𝐴(𝜑 → 𝜓) ↔ (∀𝑥 ∈ 𝐴 (𝜑 → 𝜓) ∧ ∃𝑥 ∈ 𝐴 𝜑))

This definition is used by:  dfrals2  50855  ralsd  50857  rals1d  50860  rals2d  50861  ralsex  50864  ralsbii  50866  nfrals  50869  ralrals  50873  rexrals  50874  ralsanmo  50876  ralseurals  50890