Full Idea
In Gentzen's natural deduction, the inferences are broken down into atomic steps in such a way that each step involves only one logical constant. The steps are the introduction or elimination of the logical constants.
Gist of Idea
In natural deduction, inferences are atomic steps involving just one logical constant
Source
Dag Prawitz (Gentzen's Analysis of First-Order Proofs [1974], 1.1)
Book Reference
'A Philosophical Companion to First-Order Logic', ed/tr. Hughes,R.I.G. [Hackett 1993], p.202