Natural deduction

Natural deduction is a branch of mathematical logic developed in Poland in the 1920s and 30s. It is meant to express inference rules closely related to the "natural" way of reasoning.
Spurred on by a series of seminars in Poland in 1926 by Łukasiewicz that advocated a more natural treatment of logic, Jaśkowski made the earliest attempts at defining a more natural deduction. In 1929 he first suggested in 1929 using a diagrammatic notation, and later updated his proposal in papers in 1934 and 1935.