Syllogisms with Venn diagrams

The four statement types, the must-be-true test, and the either-or case.

15 min read · Lesson 6 of 6 · Pro

The rule that decides everything

A conclusion is valid only if it is true in every arrangement that the statements allow. Not likely, not usually, not in the obvious drawing — every single one.

So the method is: try to draw a picture where the statements hold but the conclusion fails. One such picture makes the conclusion invalid.

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Take the statements as true even when they are absurd. "All stones are flowers" is a normal exam statement. Bringing in what you know about the real world is the fastest way to get these wrong.

The four statement types, as circles

All A are B. Circle A sits entirely inside circle B. Part of B may not be A.

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