logic & critical herman j. suhendraproduced by herman j. suhendra a.b. gadjah mada university...

Logic & Critical ThinkingLogic & Critical Thinking@ @ Herman J. SuhendraHerman J. Suhendra Produced by Herman J. SuhendraProduced by Herman J. Suhendra

A.B. Gadjah Mada University & M.A. University of Santo Tomas, ManilaA.B. Gadjah Mada University & M.A. University of Santo Tomas, Manila

MEETING 5Argument Part 2

Arguments - Part 1 (of 3)Arguments - Part 1 (of 3)1. Distinguishing Fact & Opinion

7. Evaluating Arguments

2. What is an Argument?

5. Deduction &Induction

6. Analyzing Arguments

8. Writing Arguments

3. Identifying Premises & Conclusions

4. What Is Not an Argument?

Remember!Remember!Before we can effectively Before we can effectively analyze and evaluate an analyze and evaluate an

argument, we need to argument, we need to understand clearly what understand clearly what

kind of argument is being kind of argument is being offered.offered.

Deduction & InductionDeduction & InductionArguments

below deductive or inductive?

Argument 1All Humans are Mortal.SBY is human.Therefore, SBY is Mortal.Argument 2All of Steven Spielberg‘s movies have been good.Therefore, Steven Spielberg‘s next movie will probably be good.

Types of Arguments:Deductive argumentsDeductive arguments are arguments in which the conclusion is

claimed or intended to follow necessarily from the premises.Inductive argumentsInductive arguments are arguments in which the conclusion is

claimed or intended to follow probably from the premises.

Key Differences: Deductive arguments claim that…Deductive arguments claim that… Inductive arguments claim that…Inductive arguments claim that… If the premises are true, then the

conclusion must be true. The conclusion follows necessarily

from the premises. The premises provide conclusive

evidence for the truth of the conclusion.

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

It is logically inconsistent to assert the premises and deny the conclusion, meaning that if you accept the premises, you must accept the conclusion.

If the premises are true, then the conclusion is probably true.

The conclusion follows probably from the premises.

The premises provide good (but not conclusive) evidence for the truth of the conclusion.

It is unlikely for the premises to be true and the conclusion false.

Although it is logically consistent to assert the premises and deny the conclusion, the conclusion is probably true if the premises are true.

Deduction & InductionDeduction & InductionThere are four tests that can be used to determine whether an argument is deductive or inductive:

1.1. The Indicator Word TestThe Indicator Word Test2.2. The Strict Necessity TestThe Strict Necessity Test3.3. The Common Pattern TestThe Common Pattern Test4.4. The Principle of Charity TestThe Principle of Charity Test

1. The Indicator Word Test1. The Indicator Word Test

The indicator word testindicator word test asks whether there are any indicator words that provide clues whether a deductive or inductive argument is being offered.Common deduction indicator words include words or phrases like necessarily, logically, it must be the case that, and this proves that.Common induction indicator words include words or phrases like probably, likely, it is plausible to suppose that, it is reasonable to think that, and it's a good bet that.In the example above, the word probably shows that the argument is inductive.

Femina is a PU student.Most PU students own laptops.

So, probably Femina owns a laptop.

The Mona Lisa, by Da Vinci, is an excellent piece of art. The LastSupper, by Da Vinci, is an excellent piece of art. The Madonna ofthe Rocks, by Da Vinci, is an excellent piece of art. So probably, all works done by Da Vinci are excellent pieces of art.

deductive or inductive?

Why?

2. The Strict Necessity Test2. The Strict Necessity Test

The strict necessity teststrict necessity test asks whether the conclusion follows from the premises with strict logical necessity. If it does, then the argument is deductive.In this example, the conclusion does follow from the premises with strict logical necessity. Although the premises are both false, the conclusion does follow logically from the premises, because if the premises were true, then the conclusion would be true as well.

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

Our national heroes are Javanese.Pangeran Diponegoro is a national hero.

Therefore, Pangeran Diponegoro is a Javanese.


Why?

3. The Common Pattern Test3. The Common Pattern Test

The common pattern testcommon pattern test asks whether the argument exhibits a pattern of reasoning that is characteristically deductive or inductive.If the argument exhibits a pattern of reasoning that is characteristically deductive, 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 eliminationArguments by elimination are arguments that seek to logically rule out various possibilities until only a single possibility remains. Arguments of this type are always deductive.

Either Djoko voted in the last election, or he didn't.Only citizens can vote. Djoko is not, and has never been, a citizen.So, Djoko didn't vote in the last election.

4. The Principle of Charity Test4. The Principle of Charity Test

In this passage, there are no clear indications whether Zeno's argument should be regarded as deductive or inductive. For arguments like these, we fall back on the principle of charity testprinciple of charity test.According to the principle of charity testprinciple of charity test, we should always interpret an unclear argument or passage as generously as possible.We could interpret Zeno's argument as deductive. But this would be uncharitable, since the conclusion clearly doesn't follow from the premises with strict logical necessity. (It is logically possible--although highly unlikely--that a 90-year-old woman who walks with a cane could climb Gunung Merbabu.) Thus, the principle of charity test tells us to treat the argument as deductive.

Ramlan: Karen told me her grandmother recently climbed Gunung Merbabu.Zeno : Well, Karen must be pulling your leg. Karen's grandmother is over 90 years old and walks with a cane.

80% of the Indonesian lives below the poverty line. Martono is an Indonesian. So probably, Martono lives below the poverty line.


Why?

If Munir was assassinated, then he died. He was assassinated. Therefore, he died.


Why?

Exercise 1Exercise 1TonyTony:: Are there any good Italian restaurants in town? Are there any good Italian restaurants in town?

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

Is Nasir’s argument deductive or

inductive? Why?

Exercise 2Exercise 2Is this argument

deductive or inductive? Why?

I wonder if I have enough cash to buy my journalism textbook I wonder if I have enough cash to buy my journalism textbook as well as my public relations and communication textbooks. Let's see, I have as well as my public relations and communication textbooks. Let's see, I have Rp200.000. My journalism textbook costs Rp65.000 and my communication Rp200.000. My journalism textbook costs Rp65.000 and my communication textbook costs Rp52.000. My public relations textbook costs Rp60.000. textbook costs Rp52.000. My public relations textbook costs Rp60.000. With taxes, that should come to about Rp190.000 Yep, I have enoughWith taxes, that should come to about Rp190.000 Yep, I have enough..

Exercise 3Exercise 3 deductive or inductive? Why?

MotherMother:: Don't give Jason that brownie. It contains walnuts, and I Don't give Jason that brownie. It contains walnuts, and I think he is allergic to walnuts. Last week he ate some oatmeal think he is allergic to walnuts. Last week he ate some oatmeal cookies with walnuts, and he broke out in a severe rash.cookies with walnuts, and he broke out in a severe rash.FatherFather:: Jason isn't allergic to walnuts. Don't you remember he ate Jason isn't allergic to walnuts. Don't you remember he ate some walnut fudge ice cream at Ferrari's birthday party last some walnut fudge ice cream at Ferrari's birthday party last spring? He didn't have any allergic reaction then.spring? He didn't have any allergic reaction then.


I went to Burger King last night and the service was horrible. The same thing happened the last time I went there. The same thing happened the time before that. Hence the service at Burger King is always horrible.


Susan is under 18. People under 18 in Indonesian cannot vote. Therefore, Susan cannot vote


Imagine a friend gave you a guinea pig to look after but forgot to tell youanything about what to feed it. You might say to yourself, 'I have a guineapig and do not know what to feed it; but I do know that my rabbit eatscarrots, and that rabbits and guinea pigs are similar. Hence, I can probablyfeed my guinea pig carrots as well'.


Shaving cream is clearly similar in colour, texture, moistness, and body towhipped cream, and I know that whipped cream is delicious on fruit salad.Hence, shaving cream is delicious on fruit salad.

Deduction & InductionDeduction & InductionTypeType DescriptionDescriptionInductiveInductiveReasoningReasoning

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 Questions to ask:

What evidence is available? What has been observed? What can be concluded from that evidence? Is that conclusion logical?

DeductiveDeductiveReasoningReasoning

Moves from conclusion to evidence for the conclusion Evaluate if the evidence is valid Includes formal logic Deductive = Conclusion Evidence Questions to ask:

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

Deductive ValidityDeductive ValidityArgument #1 :

Barbie is over 90 years old. So Barbie is over 20 years old.

Argument #2 : Barbie is over 20 years old. So Barbie is over 90 years old.

Deduction ValidityDeduction Validity• Argument #1 :

Barbie is over 90 years old. So Barbie is over 20 years old. • Argument #2 :

Barbie is over 20 years old. So Barbie is over 90 years old.

Intuitively, the conclusion of the first argument follows from the premise, Intuitively, the conclusion of the first argument follows from the premise, whereas the conclusion of the second argument does not follow from its whereas the conclusion of the second argument does not follow from its premise. But how should we explain the difference between the two arguments premise. But how should we explain the difference between the two arguments more precisely? Here is a thought : more precisely? Here is a thought :

In the first argument, if the premise is indeed true, then the conclusion cannot In the first argument, if the premise is indeed true, then the conclusion cannot be false. On the other hand, even if the premise in the second argument is true, be false. On the other hand, even if the premise in the second argument is true, there is no guarantee that the conclusion must also be true. there is no guarantee that the conclusion must also be true.

For example, Barbie could be 30 years old.For example, Barbie could be 30 years old.

DeductiveDeductiveVALID (official definition)Iff It is impossible the conclusion to be FALSE while all the premises are true.That is : There is no logically possible situation where all the premises are true and the conclusion is false at the same time.

VALID (intuitive idea)Iff the truth of premises 100% logically guarantees the truth of conclusion.It is logically NECESSARY that IF all the premises are true THEN the conclusion is also true.

INVALIDIff it is not logically NECESSARY that IF all the premises are true THENthe conclusion is also true.Iff it is logically POSSIBLE for the conclusion to be false WHILE all the Premises are true.

What do we mean by “logically possible”?What do we mean by “logically possible”?

Anything is logically possible so long as it is NOT self-contradictory.

IMPORTANT:“VALIDITY” is a logical concept: it is defined in terms of “logical possibilty”.(NOT any other kinds of possibility: Economy, political, technological, psychological, physical, or legal possibilities)

POSSIBILITY: that which is ALLOWED (i.e. not ruled out)IMPOSSIBILITY: that which is ruled out (i.e. not allowed).SOMETHING is logically possible (or logically impossible): it is allowed (or ruled out) by the laws of LOGIC.

LAW OF NON-CONTRADICTION: SELF-CONTRADICTION (i.e. “P and not-P”) IS NOT ALLOWED

DeductiveDeductive

The idea of validity provides a more precise explication of what it is for a conclusion to follow from the premises. Applying this definition, we can see that the FIRST argument above is VALID, since there is no possible situation where Barbie can be over 90 but not over 20. The SECOND argument is INVALID because there are plenty of possible situations where the premise is true but the conclusion is false. Consider a situation where Barbie is 25, or one where she is 85. The fact that these situations are possible is enough to show the argument is not VALID, or INVALID.

DeductiveDeductive

All pigs can fly. Anything that can fly can swim. So all pigs can swim.

DeductivelyDeductively

Although the two premises of this argument are false, this is actually a VALID argument. To evaluate its validity, ask yourself whether it is possible to come up with a situation where all the premises are true and the conclusion is false. (We are not asking whether there is a situation where the premises and the conclusion are all true.) Of course, the answer is 'no'. If pigs can indeed fly, and if anything that can fly can also swim, then it must be the case that all pigs can swim.

So this example tells us something:So this example tells us something:

(1) The premises and the conclusion of a valid argument can all be false.

DeductiveDeductive

Hopefully you will now realize thatHopefully you will now realize that validity validity is notis not about the about the actualactual truth or falsity of the premises or the conclusion. Validity is about truth or falsity of the premises or the conclusion. Validity is about the the logical connectionlogical connection between the premises and the conclusion. between the premises and the conclusion.

A A validvalid argument is one where the truth of the premises argument is one where the truth of the premises guarantees the truth of the conclusion, but guarantees the truth of the conclusion, but validity does not validity does not guaranteeguarantee that the premises are in fact true. All that validity tells us that the premises are in fact true. All that validity tells us is that if the premises are true, the conclusion must also be true.is that if the premises are true, the conclusion must also be true.

DeductiveDeductive

Adam loves Beth. Beth loves Cathy. So Adam loves Cathy.

DeductiveDeductiveInvalid for it is possible that the premises are true and yet the Invalid for it is possible that the premises are true and yet the conclusion is false. Perhaps Adam loves Beth but does not want conclusion is false. Perhaps Adam loves Beth but does not want Beth to love anyone else. So Adam actually hates Cathy. The mere Beth to love anyone else. So Adam actually hates Cathy. The mere possibility of such a situation is enough to show that the argument possibility of such a situation is enough to show that the argument is not valid. is not valid.

Let us call these situations Let us call these situations invalidating counterexamplesinvalidating counterexamples to the to the argument. Basically, we are defining a valid argument as an argument. Basically, we are defining a valid argument as an argument with no possible invalidating counterexamples. argument with no possible invalidating counterexamples.

Note: to sharpen your skills in evaluating arguments, it is therefore Note: to sharpen your skills in evaluating arguments, it is therefore important that you are able to discover and construct such important that you are able to discover and construct such examples.examples.

DeductiveDeductiveNotice that a counterexample Notice that a counterexample need not be realneed not be real in the sense of in the sense of being an actual situation. It might turn out that in fact that Adam, being an actual situation. It might turn out that in fact that Adam, Beth and Cathy are members of the same family and they love Beth and Cathy are members of the same family and they love each other. But the above argument is still each other. But the above argument is still invalidinvalid since the since the counterexample constructed is a possible situation, even if it is not counterexample constructed is a possible situation, even if it is not actually real. All that is required of a counterexample is that the actually real. All that is required of a counterexample is that the situation is a coherent one in which all the premises of the situation is a coherent one in which all the premises of the argument are true and the conclusion is false. argument are true and the conclusion is false.

So we should remember this :So we should remember this :

(2) An argument can be invalid even if the conclusion & the premises are all actually true.

DeductiveDeductive

All pigs are purple in colour. Anything that is purple is an animal.

So all pig are animals .

DeductiveDeductive

(3) It is possible for a VALID argument to have a true conclusion even.

when all its premises are false.

DeductiveDeductive

The concept of validity provides a more precise explication of what The concept of validity provides a more precise explication of what it is for a conclusion to follow from the premises. Since this is one it is for a conclusion to follow from the premises. Since this is one of the most important concepts, you should make sure you fully of the most important concepts, you should make sure you fully understand the definition. In giving our definition we are making a understand the definition. In giving our definition we are making a distinction between truth and validity. In ordinary usage "valid" is distinction between truth and validity. In ordinary usage "valid" is often used interchangeably with "true" (similarly with "false" and often used interchangeably with "true" (similarly with "false" and "not valid"). But here validity is restricted to only arguments and not "not valid"). But here validity is restricted to only arguments and not statements, and truth is a property of statements but not statements, and truth is a property of statements but not arguments:arguments:

So never say things like “this statement is valid” or“that argument is true”!

Exercise 2Exercise 2

Someone is sick. Someone is unhappy. So, Someone is unhappy and sick.Someone is sick. Someone is unhappy. So, Someone is unhappy and sick.

Valid or Invalid: Why?


If he loves me then he gives me some flowers. He gives me flowers. If he loves me then he gives me some flowers. He gives me flowers. So, he loves me.So, he loves me.



Beckham is famous. Beckham is a football player. Therefore, Beckham is Beckham is famous. Beckham is a football player. Therefore, Beckham is a famous football player.a famous football player.



If it rains, the street will be wet. If the streets are wet, accidents willIf it rains, the street will be wet. If the streets are wet, accidents willhappen. Therefore, accidents will happen if it rains.happen. Therefore, accidents will happen if it rains.


Deductive SoundnessDeductive Soundness

It should be obvious by now that validity is about the logical connection between the It should be obvious by now that validity is about the logical connection between the premises and the conclusion. When we are told that an argument is valid, this is not premises and the conclusion. When we are told that an argument is valid, this is not enough to tell us anything about the actual truth or falsity of the premises or the enough to tell us anything about the actual truth or falsity of the premises or the conclusion. All we know is that there is a logical connection between them, that the conclusion. All we know is that there is a logical connection between them, that the premises entail the conclusion.premises entail the conclusion.

So even if we are given a valid argument, we still need to be careful before So even if we are given a valid argument, we still need to be careful before accepting the conclusion, since a accepting the conclusion, since a validvalid argument might contain a argument might contain a false conclusionfalse conclusion. . What we need to check further is of course whether the premises are true. What we need to check further is of course whether the premises are true.

So never say things like “this statement is sound”/ “invalid” or“that argument is true”!

Deductive SoundnessDeductive SoundnessIf an argument is If an argument is validvalid, and all the premises are , and all the premises are truetrue, then it is , then it is called a called a soundsound argument. Of course, it follows from such a argument. Of course, it follows from such a definition that a sound argument must also have a true conclusion. definition that a sound argument must also have a true conclusion. In a valid argument, if the premises are true, then the conclusion In a valid argument, if the premises are true, then the conclusion cannot be false, since by definition it is impossible for a valid cannot be false, since by definition it is impossible for a valid argument to have true premises and a false conclusion in the same argument to have true premises and a false conclusion in the same situation. So given that a sound argument is valid and has true situation. So given that a sound argument is valid and has true premises, its conclusion must also be true. So if you have premises, its conclusion must also be true. So if you have determined that an argument is indeed sound, you can certainly determined that an argument is indeed sound, you can certainly accept the conclusion. accept the conclusion.

An argument that is not sound is an An argument that is not sound is an unsoundunsound argument. If an argument. If an argument is unsound, it might be that it is invalid, or maybe it has at argument is unsound, it might be that it is invalid, or maybe it has at least one false premise, or both.least one false premise, or both.

So never say things like “this statement is sound”/ “invalid” or“that argument is true”!

Deductive SoundnessDeductive Soundness

Sound: Valid + All Premises are TRUE

Unsound: Invalid + one of the premise is FALSE

Comprehension:Comprehension:1. All invalid arguments are unsound. 1. All invalid arguments are unsound. 2. All true statements are valid. 2. All true statements are valid. 3. To show that an argument is unsound, we must at 3. To show that an argument is unsound, we must at least show that some of its premises are actually false. least show that some of its premises are actually false. 4. An invalid argument must have a false conclusion. 4. An invalid argument must have a false conclusion. 5. If all the premises of a valid argument are false, then 5. If all the premises of a valid argument are false, then the conclusion must also be false. the conclusion must also be false. 6. If all the premises and the conclusion of an argument 6. If all the premises and the conclusion of an argument are true, then the argument is valid. are true, then the argument is valid. 7. All sound arguments are true. 7. All sound arguments are true. 8. Any valid argument with a true conclusion is sound.8. Any valid argument with a true conclusion is sound.

Group ActivityGroup Activity

20 min Group discussion 5 min Summarize discussion findings 15 min Group presentation & discussionThe Group leader must submit their findings in hard or soft-copy format to the lecturer and send to his email before or during the next class.

SummarySummaryDeduction and Induction

Deductive argumentsDeductive arguments are arguments in which the conclusion is claimed or intended to follow necessarily from the premises.Inductive argumentsInductive arguments are arguments in which the conclusion is claimed or intended to follow probably from the premises.

Deductively Valid and Sound

1. The premises and the conclusion of an invalid argument can all be true. 2. A valid argument should not be defined as an argument with true premises and a true conclusion. 3. The premises and the conclusion of a valid argument can all be false. 4. A valid argument with false premises can still have a true conclusion 5. A Sound argument is a valid argument with all the premises are true.

Any Questions?Any Questions?

The End – Thank You!The End – Thank You!

Failed!Failed!

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logic & critical herman j. suhendraproduced by herman j. suhendra a.b. gadjah mada university...

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