Deductive Inductive

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Evaluating Arguments


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Argument

Deductive Inductive

1. the nature of the link between premises andconclusion

2. the occurrence of special indicator words3. the form of argumentation the arguer uses


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the nature of the link betweenpremises and conclusion

Deductive A deductive argument is

one in which the conclusion

is presumed to follownecessarily from thepremises.

Assuming that premises are

true, it is absolutelyimpossible that theconclusion be false.

Inductive An inductive argument is

one in which the conclusion

is presumed to follow onlyprobably from thepremises.

Assuming that the premises

are true, it is possible, butnot probable, that theconclusion is false.


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Majority of the Ateneo students are taking up

nursing.Juan dela Cruz is studying in AteneoTherefore, Juan must be a nursing student.

All philosophy teachers are kind to theirstudents.

Jones is a philosophy teacher.Therefore, he is kind to his students.


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the occurrence of special indicatorwords

Deductive Necessarily Certainly Absolutely Definitely

Inductive Probable Improbable Plausible Implausible Likely Unlikely reasonable to conclude

Ambiguous

Must Definitely Certainly


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The sign on the candy machine reads "Out of Order. The candy machine must be broken.

The prime rib, beef stronganoff, and lobster

thermidor at the Pioneer House are allexcellent. The likely conclusion is that all theentries on the menu are excellent.

Because triangle A is congruent with triangleB, and triangle A is isosceles, it necessarilyfollows that triangle B is isosceles.


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The form of argumentation the argueruses

Deductive argument from definition categorical syllogism hypothetical syllogism disjunctive syllogism

Inductive Prediction argument from analogy inductive generalization argument from authority argument based on signs causal inference


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argument from definition

is an argument in which the conclusion isclaimed to depend merely upon the definitionof some word or phrase used in the premiseor conclusion

Because Claudia is mendacious , it follows thatshe tells lies.

Because a certain paragraph is prolix , itfollows that it is excessively wordy.


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disjunctive syllogism

A syllogism having a disjunctive statement(i.e., an "either or" statement for one orboth of its premises.

Either Gloria is persecuted for cheating duringthe elections or the judiciary is unreliable.Gloria was not persecuted.Therefore, the judiciary is unreliable.


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prediction

the premises deal with some known event in thepresent or past, and the conclusion moves beyond thisevent to some event in the relative future.

the future cannot be known with certainty; thus the

argument inductive.

Because an ITCZ has been observed to develop overEastern Mindanao, a storm will occur there in six

hours. Because certain fluctuations occurred in the prime

interest rate on Friday, the value of the dollar willdecrease against foreign currencies on Monday.


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argument from analogy Is an argument that depends on the existence of

a similarity, between two things or states of affairs.

Because of the existence of this analogy, a certaincondition that affects the better-known thing orsituation is concluded to affect the similar, lesser-known thing or situation.

Because the oil being pumped from a certain wellin Texas has a high sulfur content, the oil beingpumped from nearby wells also has a high sulfurcontent.


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inductive generalization is an argument that proceeds from the knowledge of a

selected sample to some claim about the whole group. Because the members of the sample have a certain

characteristic, it is argued that all the members of thegroup have that same characteristic.

Because three oranges selected from a certain cratewere especially tasty and juicy, all the oranges fromthat crate are especially tasty and juicy.

Because six out of a total of nine Ateneo collegestudents said that they have cars, therefore two-thirdsof the entire college population own cars.


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argument based on signs

is an argument that proceeds from theknowledge of a certain sign to a knowledge of the thing or situation that the sign symbolizes.

She wears a wedding ring, she must bemarried.

She is hot, she must be having a fever.


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causal inference

Are arguments that proceed from knowledge of acause to knowledge of the effect, or, conversely,from the knowledge of an effect to knowledge of

a cause. Because the bottle of wine had been accidentally

left in the freezer overnight, we can infer that it

had frozen (cause to effect). The chicken I ate is dry and crunchy, it definitely

had been overcooked (effect to cause).


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Argument

Deductive

Valid

Invalid

Inductive


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All human beings are mortals Ben is mortal So Ben is human

If it rained last night the streets will be wet.The streets are wet.So, it rained last night.

Since Christmas comes before New Year and Halloweencomes before New Year, it follows that Halloween comesbefore Christmas.

Either the battery is dead or there is a short in the ignitionswitch.The battery is deadTherefore, there is a short in the ignition switch.


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Argument

Deductive

Valid

Sound

Unsound

Invalid

Unsound

Inductive


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Valid Invalid

True premisesTrue

conclusion

All wines are beverages.Tuba is a wine.

Thus, Tuba is a beverage.(Sound)

All wines are beverages.Tuba is a beverage.

Thus, Tuba is a wine.(Unsound)

True premisesFalseconclusion

None exists All wines are beverages.Coke is a beverage.Thus, Coke is a wine.

(Unsound)

FalsepremisesTrueconclusion

All wines are soft drinks.Coke is a wine.Thus, Coke is a soft drink.

(Unsound)

All wines are whiskeys.Tanduay is a whiskey.Thus, Tanduay is a wine.

(Unsound)

FalsepremisesFalseconclusion

All wines are whiskeys.Coke is a wine.Thus, Coke is a whiskey.

(Unsound)

All wines are whiskeys.Coke is a whiskey.Thus, Coke is a wine.

(Unsound)


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Argument

Deductive Inductive

Weak

Strong


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Strong vs. Weak

a strong inductiveargument is aninductive argument in

which it is improbablethat the conclusion befalse given that thepremises are true.

the conclusion does infact follow probablyfrom the premises

weak inductiveargument is anargument in which the

conclusion does notfollow probably fromthe premises, eventhough it is claimed to.

the conclusion does notfollow probably fromthe premises


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All dinosaur bones discovered to this day havebeen at least 50 million years old. Therefore,probably the next dinosaur bone to be foundwill be at least 50 million years old.

All meteorites found to this day havecontained sugar. Therefore, probably the nextmeteorite to be found will contain sugar.


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Cogency and Uncogency A cogent argument is an inductive argument that

is: strong

Has all true premises if either condition is missing, the argument isuncogent.

A cogent argument is the inductive analogue of a

sound deductive argument and is what is meantby a "good" inductive argument withoutqualification.


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Argument

Deductive Inductive

Strong

Cogent

Uncogent

Weak

Uncogent

St W k


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Strong Weak

True premiseProbably trueconclusion

All previous AdZU presidents wereJesuits.Therefore, probably the next AdZUpresident will be a Jesuit.[cogent]

Several AdZU presidents wereFilipino Jesuits.Therefore, probably the next AdZUpresident will be a Filipino Jesuit.[uncogent]

True premiseProbably falseconclusion

None exist A few AdZU presidents wereAmerican Jesuits.Therefore, probably the nextpresident will be an American

Jesuit.[uncogent]

False premiseProbably trueconclusion

Most but not all AdZU presidentsare Jesuits.Therefore, all AdZU presidents areJesuits.[uncogent]

Only some of who became Adzupresidents were unmarried.Therefore, probably the next AdZUpresident will be unmarried.[uncogent]

False premiseProbably falseconclusion

All previous AdZU presidentsbecame a pope.Therefore, probably the next AdZUpresident will become a pope.

[uncogent]

A few AdZU presidents weredentists.Therefore, probably the next AdZUpresident will be a dentist.

[uncogent]

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Deductive Inductive