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The universally known Venn diagrams (actually Euler's) also trace
their historical lineage up to Llull. He was the first to represent
graphically his "concepts" by circles and link them by superposing and
intersecting them (though his aim was not to show whether they had an
intersection but to demonstrate that they had a more or less strong
affinity). (He also linked the terms of a syllogism with a
triangle, in what was later called "pons asinorum".) Llull's
circle drawings of concepts became a learning aid in J. L. Vives'
hands in the l6th century, and were perfected in the l7th by Sturm
or Leibniz -who created a whole (unpublished) logical notation out
of them- and finally, yes, Euler (in the 1760's). (Now we
call them, improperly, Venn's diagrams.)
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