[英]Material implication in intuitionistic logic
我不理解为什么在直觉逻辑中¬PVQ会导致P->Q。我理解为什么经典上是等价的,但是我不理解直觉逻辑。
---------- (id)
f, P |- f
------ (id) ---------- (Ex Falso)
P |- P f, P |- Q
--------------------- (=>L) --------- (id)
(P => f), P |- Q Q, P |- Q
------------------------------------------- (vL)
(P => f) v Q, P |- Q
---------------------- (=>R)
(P => f) v Q |- P => Q
这是阿格达的证明:
data ⊥ : Set where
⊥-elim : ∀ {w} {Whatever : Set w} → ⊥ → Whatever
⊥-elim ()
data Either (P Q : Set) : Set where
Left : P → Either P Q
Right : Q → Either P Q
material-implication : {P Q : Set} → Either (P → ⊥) Q → (P → Q)
material-implication (Left notp) p = ⊥-elim (notp p)
material-implication (Right q) p = q
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