diff --git a/library/logic/axioms/prop_decidable.lean b/library/logic/axioms/prop_decidable.lean index c1d723b04..14daeeb83 100644 --- a/library/logic/axioms/prop_decidable.lean +++ b/library/logic/axioms/prop_decidable.lean @@ -4,20 +4,17 @@ Released under Apache 2.0 license as described in the file LICENSE. Module: logic.axioms.prop_decidable Author: Leonardo de Moura + +Excluded middle + Hilbert implies every proposition is decidable. -/ import logic.axioms.prop_complete logic.axioms.hilbert open decidable inhabited nonempty --- Excluded middle + Hilbert implies every proposition is decidable - --- First, we show that (decidable a) is inhabited for any 'a' using the excluded middle theorem decidable_inhabited [instance] (a : Prop) : inhabited (decidable a) := inhabited_of_nonempty (or.elim (em a) (assume Ha, nonempty.intro (inl Ha)) (assume Hna, nonempty.intro (inr Hna))) --- Note that decidable_inhabited is marked as an instance, and it is silently used --- for synthesizing the implicit argument in the following 'epsilon' theorem prop_decidable [instance] (a : Prop) : decidable a := -epsilon (λd, true) +arbitrary (decidable a)