A tautology is a statement that is always true. A contradiction is a statement that is always false. A contingent is neither of the above (it can be true or false).

Each letter represents a statement that can be true or false. Statements can be connected with binary conjunctions. They are: • (this stands for AND. A•B is true when A and B are both true, otherwise A•B is false). v (this stands for OR. AvB is false when both A and B are false, otherwise AvB is true). ~ (this stands for NOT. ~A is false when A is true, and is true when A is false). ⊃ (IF-THEN. A⊃B stands for IF A, THEN B. A⊃B is false when A is true and B is false. Otherwise A⊃B is true). ≡ (stands for IF AND ONLY IF. A≡B is true when A and B are both true, or if they are both false. A≡B is false when one is true and the other is false).An example of a tautology is Av~A. (Example in words [Let A be the statement "It's raining"]: It's raining or it's not raining). An example of a contradiction is A•~A (example: It's raining and not raining).

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