Cut the Knot
Cut the Knot!: Mathematics: The Science of Patterns
A book excerpt, this page discusses John Venn's "simple, geometric method for checking the validity of syllogisms," with diagrams, examples, and related links.
University of St. Andrews (UK)
University of St. Andrews: John Venn
Good introduction to Venn's life and work. Explains especially his lifelong relationship to Cambridge. Includes links to further information on Venn's "probability theory."