Ideas from 'works' by Gerhard Gentzen [1938], by Theme Structure
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5. Theory of Logic / A. Overview of Logic / 2. History of Logic
11022

Gentzen introduced a natural deduction calculus (NK) in 1934

5. Theory of Logic / E. Structures of Logic / 2. Logical Connectives / a. Logical connectives
11065

The inferential role of a logical constant constitutes its meaning

11023

The logical connectives are 'defined' by their introduction rules

11213

Each logical symbol has an 'introduction' rule to define it, and hence an 'elimination' rule

6. Mathematics / B. Foundations for Mathematics / 3. Axioms for Number / g. Incompleteness of Arithmetic
10067

Gentzen proved the consistency of arithmetic from assumptions beyond arithmetic
