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# Things to know about logic - Modus Tollens If P then Q Not...

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A FEW THINGS YOU SHOULD KNOW ABOUT LOGIC An argument is a set of statements consisting of one or more premises and a conclusion asserted as following from them. A valid argument is one in which if the premises are all true, then the conclusion must be true. A veracious argument is one in which the premises are all true. A sound argument is one that is both valid and veracious. (So the conclusion of a sound argument is always true.) Two valid argument forms: Modus Ponens If P then Q. P. Therefore, Q Example: If it rained this morning and the sun has not yet come out, then the tennis courts are wet. It rained this morning and the sun has not yet come out. Therefore, the tennis courts are wet.
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Unformatted text preview: Modus Tollens If P then Q. Not Q. Therefore, not P. Example: If the earth were flat, the hull of a receding ship would not disappear before the sails. The hull of a receding ship does disappear before the sails. Therefore, the earth is not flat. Two invalid argument forms: Denying the Antecedent If P then Q. Not P. Therefore, not Q. Example: If I owned all the gold in Fort Knox, I would be wealthy. I do not own all the gold in Fort Knox. Therefore, I am not wealthy. Affirming the Consequent If P then Q. Q. Therefore, P. Example: If Jones is gay, he supports gay rights. Jones supports gay rights. Therefore, Jones is gay....
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