velleity See VOLITION. Venn diagram, a logic diagram invented by the logician John Venn in which standard form statements (the four kinds listed below) are represented by two appropriately marked overlapping circles, as follows: Syllogisms are represented by three overlapping circles, as in the examples below. If a few simple rules are followed, e.g. ‘diagram universal premises first,’ then in a valid syllogism diagramming the premises automatically gives a diagram in which the conclusion is represented. In an invalid syllogism diagramming the premises does not automatically give a diagram in which the conclusion is represented, as below. Venn diagrams are less perspicuous for the beginner than Euler diagrams. See also EULER DIAGRAM , SYLLOGIS. R.P.