Evaluating ArgumentsDeductive VS Inductive

Do You Agree with Him? Why?

Source: http://sergeicartoons.blogs.sapo.pt/arquivo/Global-warming.jpg

Must See:An Inconvenient Truth (Video).URL: http://video.google.com/videoplay?docid=2078944470709189270&q=%22Inconvenient+truth%22&hl=enFuturama explains Global Warming - as used in An Inconvenient Truth - Google Video.URL: http://video.google.com/videoplay?docid=-7826207674342179094&q=%22global+warming%22&hl=enClimate Crises (site): http://www.climatecrisis.net/

Source: http://www.globalwarmingart.com/wiki/Predictions_of_Future_Change_Gallery

Source: http://www.globalwarmingart.com/wiki/Predictions_of_Future_Change_Gallery

Source: http://www.globalwarmingart.com/wiki/Predictions_of_Future_Change_Gallery

Source: http://www.globalwarmingart.com/wiki/Predictions_of_Future_Change_Gallery

Source: http://www.globalwarmingart.com/wiki/Predictions_of_Future_Change_Gallery

Evaluating Arguments

2. What is an Argument?

1. DistinguishingFact & Opinion

3. Identifying Premises& Conclusions

4. What Is Notan Argument?

8. WritingArguments

5. Deduction &Induction

7. EvaluatingArguments 6. Analyzing

Arguments

“When asked how World War III would be fought,Einstein replied that he didn't know. But he knew how

World War IV would be fought: With sticks and stones!”

Remember!

Before we can effectively analyzeand evaluate an argument, we

need to understand clearly whatkind of argument is being

offered.

Argumentsbelow

deductive orinductive?

Deduction & Induction

Argument 1

All Humans are Mortal.

P. Ramlee is human.

Therefore, P. Ramlee is Mortal.

Argument 2

All of Deepika‘s movies have been good.

Therefore, Deepika‘s next movie will probably be good.

Types of Arguments:

Deductive arguments are arguments in which the conclusion isclaimed or intended to follow necessarily from the premises.

Inductive arguments are arguments in which the conclusion isclaimed or intended to follow probably from the premises.

Deduction & InductionKEY DIFFERENCES

Deductive arguments claim that… Inductive arguments claim that…

� If the premises are true, then theconclusion must be true.

� If the premises are true, then theconclusion is probably true.

� The conclusion follows necessarily � The conclusion follows probablyfrom the premises. from the premises.

� The premises provide conclusiveevidence for the truth of theconclusion.

� The premises provide good (butnot conclusive) evidence for thetruth of the conclusion.

� It is impossible for all the premises � It is unlikely for the premises to beto be true and the conclusion false. true and the conclusion false.

� It is logically inconsistent to assert � Although it is logically consistent tothe premises and deny theconclusion, meaning that if youaccept the premises, you mustaccept the conclusion.

assert the premises and deny theconclusion, the conclusion isprobably true if the premises aretrue.

Source: G Bassham & Co., Critical Thinking: A Student's Introduction, p.58

Deduction & Induction

There are four tests that can be used to determinewhether an argument is deductive or inductive:

• The Indicator Word Test• The Strict Necessity Test• The Common Pattern Test• The Principle of Charity Test

The Indicator Word Test

Farah is a BBA student.Most BBA students own laptops.

So, probably Farah owns a laptop.

The indicator word test asks whether there are any indicator words thatprovide clues whether a deductive or inductive argument is being offered.

Common deduction indicator words include words or phrases likenecessarily, logically, it must be the case that, and this proves that.

Common induction indicator words include words or phrases likeprobably, likely, it is plausible to suppose that, it is reasonable to thinkthat, and it's a good bet that.

In the example above, the word probably shows that the argument isinductive.

The Strict Necessity Test

Texans are architects.No architects are Democrats.So, no Texans are Democrats.

The strict necessity test asks whether the conclusion follows from thepremises with strict logical necessity. If it does, then the argument isdeductive.

In this example, the conclusion does follow from the premises with strictlogical necessity. Although the premises are both false, the conclusiondoes follow logically from the premises, because if the premises weretrue, then the conclusion would be true as well.

The Common Pattern Test

Either Bruce Lee voted in the last election, or he didn't.Only citizens can vote.

Bruce Lee is not, and has never been, a citizen.So, Bruce Lee didn't vote in the last election.

The common pattern test asks whether the argument exhibits a pattern ofreasoning that is characteristically deductive or inductive.

If the argument exhibits a pattern of reasoning that is characteristicallydeductive, then the argument is probably deductive.

If the argument exhibits a pattern of reasoning that is characteristically inductive,then the argument is probably inductive.

In the example above, the argument exhibits a pattern of reasoning called"argument by elimination.“

Arguments by elimination are arguments that seek to logically rule out variouspossibilities until only a single possibility remains. Arguments of this type arealways deductive.

The Principle of Charity Test

Ramlan: Karen told me her grandmother recently climbed Mt. Everest

Zaid : Well, Karen must be pulling your leg. Karen's grandmotheris over 90 years old and walks with a cane.

In this passage, there are no clear indications whether Zaid's argumentshould be regarded as deductive or inductive. For arguments like these,we fall back on the principle of charity test.

According to the principle of charity test, we should always interpret anunclear argument or passage as generously as possible.

We could interpret Zaid's argument as deductive. But this would beuncharitable, since the conclusion clearly doesn't follow from the premiseswith strict logical necessity. (It is logically possible--although highlyunlikely--that a 90-year-old woman who walks with a cane could climbMt. Everest). Thus, the principle of charity test tells us to treatthe argument as deductive.

Exercise 1

Tony: Are there any good Italian restaurants in town?

Nasir:Yeah, Luigi's is pretty good. I've had their Neapolitan rigatoni,their lasagne col pesto, and their mushroom ravioli. I don't thinkyou can go wrong with any of their pasta dishes.

Is Nasir’s argumentdeductive or

inductive? Why?

Exercise 2

I wonder if I have enough cash to buy my psychology textbookas well as my biology and history textbooks. Let's see, I have$200. My biology textbook costs $65 and my history textbookcosts $52. My psychology textbook costs $60. With taxes,that should come to about $190. Yep, I have enough.

Is this argumentdeductive or

inductive? Why?

Exercise 3

Mother:Don't give Sharia that brownie. It contains walnuts, and Ithink She is allergic to walnuts. Last week she ate some oatmealcookies with walnuts, and she broke out in a severe rash.

Father:Sharia isn't allergic to walnuts. Don't you remember she atesome walnut fudge ice cream at Farah's birthday party lastspring? She didn't have any allergic reaction then.

Is the Father’sargument

deductive orinductive? Why?

Deduction & InductionType Description

� Making observations, and then drawing conclusions from those observations� Moves from specific evidence to general conclusion� Conclusion must be figured out and then evaluated for validity� Inductive = Evidence Conclusion

InductiveReasoning

� Questions to ask:� What evidence is available? What has been observed?� What can be concluded from that evidence?� Is that conclusion logical?

� Moves from conclusion to evidence for the conclusion� Evaluate if the evidence is valid� Includes formal logic

DeductiveReasoning

� Deductive = Conclusion Evidence� Questions to ask:

� What is the conclusion?� What evidence supports it?� Is that evidence logical?

Any Questions?