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12th century Arabic logic diagrams: practice and theory

### Wilfrid Hodges

Our starting-point is an attempt in the early twelfth
century, by Abu al-Bakarat al-Baghdadi, to rebuild logic on the
basis of a system of line diagrams. To see what he was trying
to do, we have to reconstruct it with modern knowledge. At
present it seems (1) that he was working with a linear form of
Gergonne diagrams rather than Euler or Venn diagrams, and
(2) that he was developing an idea of Ibn Sina for replacing
proof-theoretic entailment by model-theoretic entailment.
Calculations show that the procedures he was trying to
introduce were feasible in principle but out of reach in practice,
not least given the cost of paper.