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Type a boolean expression (identifiers, parentheses, and operators 'and', 'or', '->', 'not') in the Wff area of the HTML FORM below and press the 'go' button. Experiment.


Remember, there are three possible outcomes: A Wff is either a tautology (always true), a contradiction (always false), or satisfiable (sometimes true, sometimes false).


  1. What kind of Wff leads to the greatest expansion when it is converted into CNF?
  2. Ditto DNF?
  3. Some advertising relies on the following kind of "reasoning":
    People who are (rich | powerful | attractive | etc.) buy XXX, therefore if you buy XXX you will become (rich | powerful | attractive | etc.).
    • Is this kind of "reasoning" valid?
    • Consider: People who are [whatever] buy food;
    • Formulate the advertising "reasoning" as a Wff and use the FORM to see the true situation.
  4. Give a related but different kind of reasoning with a different outcome.


  • Read the [propositional logic] page to learn more about the algorithm used on the HTML FORM above.
  • The programming language Prolog is based on the more powerful Predicate Logic. There is a demonstration Prolog interpreter [here...].
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