MODR 2640 Lecture Notes - Logical Form, Modus Ponens, Modus Tollens
19 Nov 2012
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2 important differences between truth tables and proofs: truth tables are mechanical-- proofs require thought, truth tables always get a definite answer-- proofs may not yield an answer. 3 things need to know: substitution instance, rules, and how to use rules. Variables-- lower case p, q, r; can be replaced by any statement. An argument is a substitution instance of a form if you can get from the form to the argument just by replacing variables with statements. Valid argument forms: modus ponens p q p. Don"t confuse m. p. with the invalid form: p q p. Don"t confuse m. t. with (invalid) denying the antecedent: p q. ~q: disjunctive syllogism (d. s. ) p v q. P: hypothetical syllogism (h. s. ) p q q r q r. P r: simplification p q p q. P v q: constructive dilemma p q r s p v r. Suppose you want to infer a line c. if there are previous lines, p.
