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October5_Lecture4_Class5_Chap3AndFourDeductiveArguementsDiagramingArguments.docx

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Department
Social Sciences and Humanities
Course
SSH 105
Professor
Andrew Hunter
Semester
Fall

Description
Critical Thinking Lecture Four October 5, 2012 Chapter 3 Deductive Argument Patterns Diagramming Arguments Deductive Argument Patterns - There are some common patterns shared by many deductive arguments - They form a frame that is common to many arguments - Understanding some basic argument patterns help to determine (a) whether an argument is deductive (b) whether is it valid or invalid - Many of these patterns involve two kids of statements Conditionals (if) & Disjunctions (either or) Conditional Statements A conditional statement is a statement of the form If p, then q Examples: - If it rains, then the picnic will be cancelled - If Jones didn’t commit the murder, the butler did Conditionals are compound statements composed of two parts: The antecendent – what follows the word “if” The consequent – what follows the word “then” The antecendent always a expresses a sufficient condition for the consequent The consequent expresses a necessary condition (not on quiz two) Disjunctive Statement A disjunctive statement is a statement of the form Either p or q. Examples Either the picnic was cancelled or it rained Either Jones commited the murder or the butler didi Disjunctions are compound statements composed of two parts called the disjuncts. Some valid conditional argument patterns 1. Affirming the antecedent (Modus Ponens): If p, then q p. Therefore q. Example: (1) If the conservatives won the election, then Stepehen Harper is the new Prime Minister (antecedent) (2) The Conservatives won the election Therefore, (3) Stepehen Harper is the new Prime Minister (from premise (1) and premise (2) by Modus Ponens) 2. Denying the consequent (Modus Tallens) If p, then q. Not q. Therefore, not p. Example: (1) If the liberals won the election, then Paul Martin is the new Prime Minister (2) Paul Martin is not the new Prime Minister (3) The liberals did not win the election (From Premise (1) and premise (2) by Modus Tallens) 3. Hypothetical Syllogism If p, then q. If q, then r. Therefore, if p, then r Example: (1) If the conservatives won the election, then Stephen Harper is the new Prime Minister (2) If Stephen Harper is the new Prime Minister, then someone from Alberta is the new Prime Minister. Therefore, (3) If the conservatives won the election, then someone from Alberta is the new Prime Minister (from Premise 1 and Premise 2 by Hypothetical Syllogism) Some Invalid Conditional Argument Patterns 4. Denying the Antecedent If p, then q Not P. Therefore not q. Example (1) If Einstein invented the computer, then he’s a genius (2) Einstein did not invent the computer Therefore, (3) He’s not a genius (From premise (1) and premise (2) by ???) 5. Affirming the Consequent If p, then q. q. Therefore p. Example (1) If Einstein invented the computer then he’s a genius (2) Einstein is a genius Therefore, (3) He invented the computer. (From Premise (1) and Premise (2) by ?? A valid disjunctive ar
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