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Deductive Arguments-1 • Differ from the sorts of non-deductivearguments thatalluncertain in onewayoranotherin which a large number of true premises do not guarantee the truth of the conclusion. Becauseconclusiongoesbeyondthepremise. (exp.relying on authority) • Validdeductivearguments , however, areargumentsthatthetruth of whosepremisesguranteesthetruth of theirconclusion.
Deductive Arguments-2 Example: Iftherearenochancefactors in chess, thenchess is a game of pureskill(premise1). Therearenochancefactors in chess(premise 2). Therefore, chess is a game of pureskill.(conclusion) • Ifthesepremisesaretrue, then it mustalso be truethatchessgame is a game of pureskill.
Deductive Arguments-3 • In real life, of course, we can't always be sure of our premises.Still, when strong premises can be found, deductive forms arevery useful. • And even when the premises are uncertain, deductiveforms offer an effective way of organizing an argument,especially an argumentative essay.
6 commondeductiveforms • ModusPonens(themode of putting) • Modus Tollens( themode of taking) • Hypothetical Syllogism • Disjunctive Syllogism • Dilemma • Reductio ad absurdum(reduction toabsurdity)
1) Modus Ponens (the mode of putting)-cont. Using the letters p and q to stand for sentences, the simplestvalid deductive form is If [sentence p] then [sentence q]. [Sentence p].Therefore, [sentence q]. Or, more briefly: If p then q. p. Therefore, q.
Modus Ponens (the mode of putting) If optimists are more likely to succeed than pessimists, thenyou should be an optimist. Optimists are more likely to succeed than pessimists. Therefore, you should be an optimist. Note: To develop this argument, you must explain and defend both ofits premises, and they require quite different arguments
2. ModusTollens( themode of taking) If p then q. Not-q. Therefore, not-p. If the dog did not know the visitor well, then the dog wouldhave barked. The dog did not bark. Therefore, the dog knew the visitor well.
3.Hypothetical Syllogism If s then r. If r then q. Therefore, if s then q. If you study other cultures, then you realize the variety ofhuman customs. If you realize the variety of human customs, then you questionyour own customs. Therefore, if you study other cultures, then you question yourown customs.
4.Disjunctive Syllogism m or i. Not-m. Therefore, i. Either we hope for progress by improvingmorals, or we hopefor progress by improving intelligence. We can't hope for progress by improving morals. Therefore, we must hope for progress by improving intelligence. (Bertrand Russell)
5.Dilemma • A choice between two optionsboth of which have bad consequences Either we go to Bodrum or we go Çeşme forthenewyear’s eve. If we go to Bodrum, then we'll havecold. If we go Çeşme, then we'll alsohavecold. Therefore, we'll havecold.
6.Reductio ad absurdum (reductiontoabsurdity) • Establish their conclusions by showingthat assuming the opposite leads to absurdity: to a contradictoryor silly result. To prove: p. Assume the opposite: Not-p. Argue that from the assumption we'd have to conclude: q. Show that q is false (contradictory, silly, "absurd"). Conclude: p must be true after all.
Suppose the world has a Creator like a house does. Now, whenhouses are not perfect, we know who to blame: the carpenters andmasons who created them. But the world is also not whollyperfect. Therefore, it would seem to follow that the Creator of theworld is not perfect either. But you would consider this conclusionabsurd. The only way to avoid the absurdity, however, is toreject the supposition that leads to it. Therefore, the world doesnot have a Creator in the way a house does.(David Hume)
To prove: The world does not have a Creator in the way a housedoes. Assume the opposite: The world does have a Creator in theway a house does. Argue that from the assumption we'd have to conclude: TheCreator is imperfect (because a house is imperfect). But: God cannot be imperfect. Conclude: The world does not have a Creator in the way ahouse does.
Fallacies • Fallacies are misleading arguments. Many of them are sotempting, and therefore so common, they even have their ownnames. • Tocall something a fallacy is usuallyonly another way of saying that it violates one of the rules forgood arguments.
The Two Great Fallacies • Generalizingfromincompleteinformation: Othercauses may still be more likely. • Overlooking alternatives: Other possibilities exist. Try to increasethe number of options you consider, not narrow them.
Some Classical Fallacies Ad hominem : attacking the person of an authority rather thanhis or her qualifications. Ad ignorantiam (appealtoignorance):arguing that a claim istrue just because it has not been shown to be false. ‘I do not have much information on this except the generalstatement of the agency that there is nothing in the files todisprove his Communist connections.’
Ad misericordiam (appeal to pity):appealing to pity as anargument for special treatment. Pity is not always a bad argument, but it is certainly inappropriatewhen objective evaluation is called for. ‘I know I failedfrom every exam, but if I don't pass this course, I'llhave to retake it in summer school. You have to let me pass!’
Ad populum: Appealing to the emotions of a crowd; also,appealing to a person to go along with the crowd. No reasons are offered to show that "everybody"is an informed or impartial source. Affirming the consequent: a deductive fallacy If i then l. l- Therefore, i. When the roads are icy, the mail is late. The mail is late. Therefore, the roads are icy.
begging the question:implicitly using your conclusion as apremise. ‘God exists because it says so in the Bible, which I know is truebecause God wrote it, after all!’ complexquestion: "Willyou follow your conscience instead of your pocketbook anddonate to the cause? "
False cause: a questionable conclusion aboutcause and effect.
Denying the antecedent: If i [ antecedent] then l [consequence] Not-i. Therefore, not-l. When the roads are icy, the mail is late. The roads are noticy. Therefore, the mail is notlate. Note : Overlooks alternativeexplanationsforbeinglate.
FalseDilemma: reducing the options you consider to just two.
poisoning the well: using loaded language to undermineanargument before even mentioning it. ‘ No sensitive person thinks that. . .’ post hoc, ergo propter hoc:(after this, therefore becauseof this): assuming causation too readily on the basis ofmere succession in time. Note: majorfactor vs. triggerfactor
red herring: introducing an irrelevant or secondary subjectand thereby diverting attention from the mainsubject.
