[New Entry by Francis Jeffry Pelletier and Allen Hazen on October 29, 2021.]
‘Natural deduction’ designates a type of logical system described initially in Gentzen (1934) and Jaśkowski (1934). It also designates the type of reasoning that these logical systems embody. A fundamental part of natural deduction, and what (according to most writers on the topic) sets it apart from other proof methods, is the notion of a “subproof”. Although formalisms differ, an underlying idea is that one is able to “make an assumption A and see that it leads to conclusion X”, and then conclude that…