How to prove excluded middle is irrefutable in Coq?

Question

I was trying to prove the following simple theorem from an online course that excluded middle is irrefutable, but got stuck pretty much at step 1:

Theorem excluded_middle_irrefutable: forall (P:Prop), ~~(P \/ ~ P).
Proof.
  intros P. unfold not. intros H.

Now I get:

1 subgoals
P : Prop
H : P \/ (P -> False) -> False
______________________________________(1/1)
False

If I apply H , then the goal would be P \\/ ~P , which is excluded middle and can't be proven constructively. But other than apply , I don't know what can be done about the hypothesis P \\/ (P -> False) -> False : implication -> is primitive, and I don't know how to destruct or decompose it. And this is the only hypothesis.

My question is, how can this be proven using only primitive tactics ( as characterized here , ie no mysterious auto s)?

Thanks.

Answer 1

I'm not an expert on this subject, but it was recently discussed on the Coq mailing-list . I'll summarize the conclusion from this thread. If you want to understand these kinds of problems more thoroughly, you should look at double-negation translation .

The problem falls within intuitionistic propositional calculus and can thus be decided by tauto .

Theorem excluded_middle_irrefutable: forall (P:Prop), ~~(P \/ ~ P).
  tauto.
Qed.

The thread also provides a more elaborate proof. I'll attempt to explain how I would have come up with this proof. Note that it's usually easier for me to deal with the programming language interpretation of lemmas, so that's what I'll do:

Theorem excluded_middle_irrefutable: forall (P:Prop), ~~(P \/ ~ P).
  unfold not.
  intros P f.

We are asked to write a function that takes the function f and produces a value of type False . The only way to get to False at this point is to invoke the function f .

 apply f.

Consequently, we are asked to provide the arguments to the function f . We have two choices, either pass P or P -> False . I don't see a way to construct a P so I'm choosing the second option.

  right.
  intro p.

We are back at square one, except that we now have a p to work with! So we apply f because that's the only thing we can do.

  apply f.

And again, we are asked to provide the argument to f . This is easy now though, because we have a p to work with.

  left.
  apply p.
Qed. 

The thread also mentions a proof that is based on some easier lemmas. The first lemma is ~(P /\\ ~P) .

Lemma lma (P:Prop) : ~(P /\ ~P).
  unfold not.
  intros H.
  destruct H as [p].
  apply H.
  apply p.
Qed.

The second lemma is ~(P \\/ Q) -> ~P /\\ ~Q :

Lemma lma' (P Q:Prop) : ~(P \/ Q) -> ~P /\ ~Q.
  unfold not.
  intros H.
  constructor.
  - intro p.
    apply H.
    left.
    apply p.
  - intro q.
    apply H.
    right.
    apply q.
Qed.   

These lemmas suffice to the show:

Theorem excluded_middle_irrefutable: forall (P:Prop), ~~(P \/ ~ P).
  intros P H.
  apply lma' in H.
  apply lma in H.
  apply H.
Qed.