Full Idea
Natural deduction adopts for → as rules the Deduction Theorem and Modus Ponens, here called →I and →E. If ψ follows φ in the proof, we can write φ→ψ (→I). φ and φ→ψ permit ψ (→E).
Clarification
'I' means introduction, and 'E' means elimination
Gist of Idea
Natural deduction rules for → are the Deduction Theorem (→I) and Modus Ponens (→E)
Source
David Bostock (Intermediate Logic [1997], 6.2)
Book Reference
Bostock,David: 'Intermediate Logic' [OUP 1997], p.248
A Reaction
Natural deduction has this neat and appealing way of formally introducing or eliminating each connective, so that you know where you are, and you know what each one means.
Related Idea
Idea 13755 Excluded middle is an introduction rule for negation, and ex falso quodlibet will eliminate it [Bostock]