Theory Drinker

(*  Title:      HOL/Examples/Drinker.thy
    Author:     Makarius
*)

section ‹The Drinker's Principle›

theory Drinker
  imports Main
begin

text ‹
  Here is another example of classical reasoning: the Drinker's Principle says
  that for some person, if he is drunk, everybody else is drunk!

  We first prove a classical part of de-Morgan's law.
›

lemma de_Morgan:
  assumes "¬ (∀x. P x)"
  shows "∃x. ¬ P x"
proof (rule classical)
  assume "∄x. ¬ P x"
  have "∀x. P x"
  proof
    fix x show "P x"
    proof (rule classical)
      assume "¬ P x"
      then have "∃x. ¬ P x" ..
      with ‹∄x. ¬ P x› show ?thesis by contradiction
    qed
  qed
  with ‹¬ (∀x. P x)› show ?thesis by contradiction
qed

theorem Drinker's_Principle: "∃x. drunk x ⟶ (∀x. drunk x)"
proof cases
  assume "∀x. drunk x"
  then have "drunk a ⟶ (∀x. drunk x)" for a ..
  then show ?thesis ..
next
  assume "¬ (∀x. drunk x)"
  then have "∃x. ¬ drunk x" by (rule de_Morgan)
  then obtain a where "¬ drunk a" ..
  have "drunk a ⟶ (∀x. drunk x)"
  proof
    assume "drunk a"
    with ‹¬ drunk a› show "∀x. drunk x" by contradiction
  qed
  then show ?thesis ..
qed

end