BBI 3420 / 3436ARGUMENTS: Deduction and Induction
OverviewExamining deductive and inductive arguments.
Telling the difference between the two.
Different kinds of each argument form.
Types of fallacies
Types of argumentsDeductive arguments
An argument in which it is impossible for a conclusion to be false if its premises are true.
An argument in which it is improbable for the conclusion to be false if its premises are true.
Deductive ReasoningStarts with a general rule (a premise) which we know to be true. Then, from that rule, we make a true conclusion about something specific.
Deductive ReasoningThe process of reasoning from known facts to conclusions. When you reason deductively, you can say therefore with certainty. If your facts were firm to begin with, then your conclusions will also be firm.The conclusion claims to follow necessarily from the premises.Example:Socrates is a man.All men are mortal.Therefore, Socrates is mortal.
A deductive argumentis one in which it is impossible for the premises to be true but the conclusion false. It is supposed to be a definitive proof of the truth of the claim (conclusion). Premise All men are mortal. Premise Socrates was a man.Conclusion Socrates was mortal.If the premises are true (and they are), then it simply isn't possible for the conclusion to be false. If you have a deductive argument and you accept the truth of the premises, then you must also accept the truth of the conclusion.
Deductive Reasoning IsDeductive reasoning is when you start from things you assume to be true, and draw conclusions that must be true if your assumptions are true. For ExampleAll dogs have a tail.Benji is a dog.Therefore Benji has a tail.
A deductive argument
Deductive ReasoningExample:Smith owns only blue pants and brown pants. Smith is wearing a pair of pants today. So, Smith is wearing either blue or brown pants today.
Say thisNot thisGravity makes things fall. The apple that hit my head was due to gravity.The apple hit my head. Gravity works!They are all like that -- just look at him!Look at him. They are all like that.Toyota make wonderful cars. Let me show you this one.These cars are all wonderful. They are made by Toyota, it seems.There is a law against smoking. Stop it now.Stop smoking, please.
Who is known for using Deductive Reasoning?
Sherlock HolmesSherlock Holmes would use deductive reasoning to help solve crimes.
Andrew AultExample:Sherlock Holmes could help solve a mystery by making inferences. If Holmes saw a pack of cigarettes by a victim (the victim did not smoke), Holmes can make the assumption that the killer is a smoker.
HoweverDeductive reasoning may not be the most accurate way of solving a problem, cause we all know that assumptions can be wrong.
Other faults of deductive reasoningAll Graduates of M.I.T. are EngineersGeorge is not from M.I.T.Therefore George is not an Engineer
Everybody from Texas is a cowboyScott is from TexasScott is a cowboy
What professions do you think commonly use Deductive Reasoning?
Why?These professions tend to ask a lot of questions to try to solve problems or to prove a point.Often they would have to make assumptions to solve problems.They would use rules and widely accepted beliefs to prove their argument.
An attorney states that his client is innocent because the crime victim was hit by a car. Since his client does not have a license. He can deduce that his client is innocent.
Deductive ReasoningFrom vague
Inductive ArgumentThe process of going from observations to conclusions. This type of conclusion is sometimes called an inference. Conclusion claims to follow probably from the premises.Example:Socrates was Greek. Most Greeks ate fish. Therefore, Socrates probably ate fish.
Inductive ReasoningObserving that something is true many times, then concluding that it will be true in all instancesUsing the data to make a prediction
Inductive ReasoningFrom specific
An inductive argumentis one in which the premises are supposed to support the conclusion.If the premises are true, it is unlikely that the conclusion is false. The conclusion probably follows from the premises. Premise Socrates was Greek.Premise Most Greeks eat fish.Conclusion Socrates ate fish.Even if both premises are true, it is still possible for the conclusion to be false (maybe Socrates was allergic to fish). Words which tend to mark an argument as inductive include probably, likely, possibly and reasonably.
A inductive argumentTrue PremiseTrue PremiseProbablyTrue Conclusion
Inductive ReasoningExample:January has been cold here in Siberia. Today is January 14, so it is going to be another cold day in Siberia.
Say thisNot thisLook at how those people are behaving. They must be mad.Those people are all mad.All of your friends are good. You can be good, too.Be good.The base costs is XXX. The extras are XXX, plus tax at XXX. Overall, it is great deal at YYY.It will cost YYY. This includes XXX for base costs, XXX for extras and XXX for tax.
Deductive ArgumentPremise: Everything made of copper conducts electricity.Premise: This wire is made of copper.Conclusion: This wire will conduct electricity.
Weaknesses in Inductive ReasoningThere are two possible weaknesses in inductive generalizations. 1) the sample may not be representative of the population it is drawn from. Is the sample representative?2) The sample may be too small, and thus there is a second question we should ask:Is the sample large enough?If the sample is unrepresentative or too small then the premises will be inadequate to support the conclusion.
Weaknesses in Inductive ReasoningSampling: ExamplesJust one observation of the effect of cold metal on a human tongue is enough for most kids to form a good generalization.
Similarly, we need not observe the case history of every smoker who has ever lived in order to conclude that smoking is a health hazard.
On the other hand, someone who concludes that all the good ones are taken on the basis of two bad dates might sensibly be advised to keep looking.
Inductive argumentThe local branch of Wachovia Bank was robbed yesterday. Jenny needed money to pay off her gambling debts. She just bought a gun two days ago, and I saw her hanging around the local Wachovia Bank yesterday morning. Today the bookies goons stopped looking for Jenny. So Jenny robbed Wachovia Bank yesterday.
Deductive ArgumentPremise: All turtles have shells.Premise: The animal I have captured is a turtle.Conclusion: I conclude that the animal in my bag has a shell.
Deductive ArgumentSherlock Holmes and Watson were on a camping trip. They had gone to bed and were lying there looking up at the sky. Holmes said, Watson, look up. What do you see?I see thousand of stars.And what does that mean to you?I guess it means we will have another nice day tomorrow. What does it mean to you, Holmes?To me, it means someone has stolen our tent. Deductive reasoning drives you to a conclusion based on known facts.
Inductive argumentPremise: Tonya is seen walking from her car to her home with a set of golf clubs.Premise: Tonyas husband Jeff loves golf and tomorrow is his birthday.Conclusion: Tonya has bought the set of golf clubs for Jeff.
Inductive reasoning depends on human observation.Tonya, after all, may be borrowing the golf clubs. Or she may have taken up golf herself! You wouldnt know unless you observed carefully, and even then, you would have to describe your conclusion as probable but not firm.
Deductive argumentThe cut-off date for swim camp registration is June 15. After that date, applicants go on a wait list - no exceptions allowed. You have missed the cut-off to date to register by two days. You wont be registered and your name will go on the waiting list.
How do we tell inductive from deductive?The distinction between inductive and deductive arguments is based on the strength of an arguments inferential claim.An inferential claim is based on a certain reasoning process it is the relationship between the premises and conclusion of an argument.But the strength of a claim is hardly ever stated outright, so we have to evaluate it.Three criteria for measuring an arguments strength:1) The occurrence of special indicator words.2) The actual strength of the inferential link between the premises and conclusion.3) The form of argumentation used by the person making the argument.
Certain indicator words lean more towards inductive and some lean towards deductive. But theyre not always accurate. Pay attention to the context of the argument.Example: The word probably tends to be used in inductive arguments, and words like therefore and necessarily tend to lean towards deductive arguments.
Nadia is a B.A (English) student.Most B.A. (English) students own laptops.So, probably Nadia owns a laptop.The indicator 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