11022 | Gentzen introduced a natural deduction calculus (NK) in 1934 |

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 |

13832 | Natural deduction shows the heart of reasoning (and sequent calculus is just a tool) |

10067 | Gentzen proved the consistency of arithmetic from assumptions beyond arithmetic |