1.  (¬α → ¬α) → ((¬α → α) → α)         A¬
2.   ¬α → ¬α                           Proposizione 3.1
3.  (¬α → α) → α                       MP(1,2)
4.   ¬α → α                            ipotesi
5.   α                                MP(3,4)                          

   ¬α → α       [¬α]1
  -----------------------------------
                α              [¬α]1
             -----------------------------
                      ⊥
                     -------- [1]
                       α