Full Idea
Gentzen introduced a natural deduction calculus (NK) in 1934.
Clarification
'Natural deduction' is all rules and few axioms
Gist of Idea
Gentzen introduced a natural deduction calculus (NK) in 1934
Source
report of Gerhard Gentzen (works [1938]) by Stephen Read - Thinking About Logic Ch.8
Book Reference
Read,Stephen: 'Thinking About Logic' [OUP 1995], p.228