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Chapter 3

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Modes Of Reasoning
MODR 1770
Glen Hoffman

MODR 1770 Oct 15/12 CH 3 – Making Sense of Arguments Lecture + Notes D EVISING AN ARGUMENT  To show that a statement/claim is worthy of acceptance © E VALUATING AN ARGUMENT  To see whether the argument shows that the statement (conclusion) is worthy of acceptance 2 forms of arguments DEDUCTIVE  Provides logically conclusive, absolute support for conclusion  Valid: if premises are true, conclusion must be true o Deductively valid arguments are truth-preserving; conclusion follows logically, order of premises makes no difference  Sound: premises are true.  Invalid: conclusion doesn’t follow logically from the premises (regardless of order of premises) o Argument w/ true conclusion can be invalid if conclusion isn’t supported by the premises  Valid deductive argument; conclusion is false; thus 1 of the premises is false INDUCTIVE Provides probable (not conclusive) support for conclusion Cogent: has true premises LOGICAL STRUCTURE Refers to the construction of an argument; the way the premises and conclusion fit together *Persuading is not the same as reasoning with crit. thinking; persuading: influence opinions by using words to appeal to their ego, gullibility, greed, anger, etc; emotional language, psychological ploys, lies, etc. Does not show that a belief is true/warranted. Crit. thinking reasoning can be psychologically compelling but the 2 have different functions. MODR 1770 Oct 15/12 Judging arguments 1. Find conclusion then premises of argument. 2. If the premises are true, the conclusion MUST be true? (Yes: deductive) a. Check to see if it’s sound (valid). 3. If the premises are true, is the conclusion PROBABLY true? (Yes: inductive) a. Check to see if it’s cogent (strong). 4. Is the argument intended to offer conclusive/probable support but fails? (Invalid/weak argument; determine what kind of argument is intended) a. If form shows it’s inductive/deductive, assume it’s intended b. If indicator words (form shows no clues) shows it’s inductive/deductive, assume it’s intended i. Deductive: absolutely, necessarily, certainly ii. Inductive: plausible that, probably, odds/chances are F INDING MISSING PARTS  Some premises are implicit/assumed and left out  Often these are controversial and must be examined  Find an unstated premise when something essential is missing that’s not a common sense generally accepted assumption.  Usually left out/downplayed to make an argument seem stronger. T O MAKE SURE INVESTIGATION OF IMPLICIT PREMISES IS THOROUGH & REASONABLE 1. Search for credible premise that would make argument valid & provide link between premise(s) & conclusion. a. Is the most plausible & best fits with author’s intent (Principle of Charity (in interpretation) b. When we find someone’s meaning unclear, should attempt to interpret it in a way that makes sense 2. (If step 1 doesn’t work) Search for credible premise that would make argument as strong as possible & fits with the principle of charity 3. Evaluate reconstructed argument. MODR 1770 Oct 15/12 Deductive Argument Patterns INVOLVE 2 KINDS OF STATEMENTS : Conditional {if-then}  If p, then q  Compound statements composed of 2 parts: o Antecedent – follows “if” o Consequent – follows “then” Disjunctive statement  Either p or q  Compound statement composed of 2 disjuncts A FFIRMING THE A NTECEDENT (M ODUS PONENS )  Valid; deductive, conditional. If p then q. P. Therefore q.  Not necessarily sound (premises could be false) D ENYING
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