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Metamath Proof Explorer

Theorem eupick

Description: Existential uniqueness "picks" a variable value for which another wff is true. If there is only one thing x such that ph is true, and there is also an x (actually the same one) such that ph and ps are both true, then ph implies ps regardless of x . This theorem can be useful for eliminating existential quantifiers in a hypothesis. Compare Theorem *14.26 in WhiteheadRussell p. 192. (Contributed by NM, 10-Jul-1994)

		Ref	Expression
	Assertion	eupick	$⊢ (\exists! x φ \land \exists x (φ \land ψ)) \to (φ \to ψ)$

Proof

Step	Hyp	Ref	Expression
1		eumo	$⊢ \exists! x φ \to \exists^{*} x φ$
2		mopick	$⊢ (\exists^{*} x φ \land \exists x (φ \land ψ)) \to (φ \to ψ)$
3	1 2	sylan	$⊢ (\exists! x φ \land \exists x (φ \land ψ)) \to (φ \to ψ)$