Help with the natural deduction proof to the sequent ⊢ (φ → ((¬φ) → ψ))

Question

I am working through Chriswell and Hodge's Mathematical Logic and am confused by the answer to the question 2.6.2 (b).

In particular why

  φ        (¬φ)
──────    ────── (¬E)
        ⊥
────────────────── (RAA)
        ψ

is a valid derivation of ψ.