在 Coq 中如何证明或证伪 `forall (PQ : Prop), (P -> Q) -> (Q -> P) -> P = Q.`？

Question

I want to prove or falsify forall (PQ : Prop), (P -> Q) -> (Q -> P) -> P = Q. in Coq. Here is my approach.

Inductive True2 : Prop :=
 | One : True2
 | Two : True2.

Lemma True_has_one : forall (t0 t1 : True), t0 = t1.
Proof.
  intros.
  destruct t0. destruct t1.
  reflexivity.
Qed.

Lemma not_True2_has_one : (forall (t0 t1 : True2), t0 = t1) -> False.
Proof.
  intros.
  specialize (H One Two).
  inversion H.

But, inversion H does nothing. I think maybe it's because the coq's proof independence (I'm not a native English speaker, and I don't know the exact words, please forgive my ignorance), and coq makes it impossible to prove One = Two -> False. But if so why has to coq eliminate the content of a proof?

Without the above proposition, I can't prove the followings or their negations.

Lemma True_neq_True2 : True = True2 -> False.

Theorem iff_eq : forall (P Q : Prop), (P -> Q) -> (Q -> P) -> P = Q.

So my question is:

1. How to or is that possible to prove or falsify forall (PQ : Prop), (P -> Q) -> (Q -> P) -> P = Q. in Coq?
2. Why inversion H does nothing; does it's because the coq's proof independence, and if so, why does Coq waste energy in doing this.

Answer 1

1. The principle you're mentioning, forall PQ : Prop, (P <-> Q) -> P = Q , is usually known as propositional extensionality . This principle is not provable in Coq's logic, and originally the logic had been designed so that it could be added as an axiom with no harm. Thus, in the standard library ( Coq.Logic.ClassicalFacts ), one can find many theorems about this principle, relating it to other well-known logical principles of classical reasoning. Surprisingly, it wasrecently found out that Coq's logic is incompatible with this principle, but for a very subtle reason. This is considered a bug, since the logic had been designed so that this could be added as an axiom with no harm. They wanted to fix this problem in the new version of Coq, but I don't know what the current status of that is. As of version 8.4, propositional extensionality is inconsistent in Coq.

   In any case, if this bug is fixed in future versions of Coq, it should not be possible to prove nor disprove this principle in Coq. In other words, the Coq team wants this principle to be independent of Coq's logic.

2. inversion H doesn't do anything there because the rules for reasoning about proofs (things whose type is a Prop ) are different from the ones for reasoning about non-proofs (things whose type is a Type ). You may know that proofs in Coq are just terms. Under the hood, inversion is essentially constructing the following term:

    Definition true_not_false : true <> false := fun H => match H in _ = b return if b then True else False with | eq_refl => I end.

   If you try to do the same with a version of bool in Prop , you get a more informative error:

    Inductive Pbool : Prop := | Ptrue : Pbool | Pfalse : Pbool. Fail Definition Ptrue_not_Pfalse : Ptrue <> Pfalse := fun H => match H in _ = b return if b then True else False with | eq_refl => I end. (* The command has indeed failed with message: *) (* => Error: *) (* Incorrect elimination of "b" in the inductive type "Pbool": *) (* the return type has sort "Type" while it should be "Prop". *) (* Elimination of an inductive object of sort Prop *) (* is not allowed on a predicate in sort Type *) (* because proofs can be eliminated only to build proofs. *)

   Indeed, one of the reasons for this is that Coq was designed to be compatible with another principle called proof irrelevance (I think that's what you meant by "proof independence").