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Reason & Argument Lecture 2.

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Presentation on theme: "Reason & Argument Lecture 2."— Presentation transcript:

1 Reason & Argument Lecture 2

2 Lecture Synopsis Making Arguments Explicit Validity & Soundness
Counter-Examples (& a word about Induction)

3 Recap Last time: Key Concepts
‘Argument’: a statement, along with some supposed reasons for accepting the statement. ‘Conclusion’: the statement for which reasons have supposedly been presented. ‘Premises’: the claims which are supposed to be reasons for accepting the argument’s conclusion.

4 Recap Spotting premises & conclusions:
They don’t always come in the same order. ‘Indicators’ Premises: for, since, because, due to the fact that, etc. Conclusions: so, hence, thus, therefore, it follows that But be careful! These can indicate other connections.

5 (1) Making Arguments Explicit
Often arguments don’t come neatly packaged as premises and conclusions: Suppressed or Un-stated Premises Implicit Conclusions Missing sub-conclusions Extraneous Information

6 (1) Making Arguments Explicit
Procedure How to reconstruct an argument Identify premises and a conclusion. Eliminate extraneous material. Fill in suppressed premises. Fill in suppressed (sub-)conclusions. Do all this even if your ultimate goal is to dispute the argument. (The ‘Principle of Charity’)

7 (1) Making Arguments Explicit
Examples (S1) Wealthy ancient Greeks always had siblings. (S2) So Plato had a sibling.

8 (1) Making Arguments Explicit
Examples Let’s try that again: (S1) Wealthy ancient Greeks always had siblings. (S3*) Plato was a wealthy ancient Greek. (S2) So Plato had a sibling.

9 (1) Making Arguments Explicit
Examples (S1) Until the 1960s, men with long hair weren’t allowed to enter Disneyland. (S2) So if Bob visited Disneyland in 1955, he didn’t get in; although (S3) Bob was actually living in Selby at the time.

10 (1) Making Arguments Explicit
Examples Let’s try that again: (S1) Until the 1960s, men with long hair weren’t allowed to enter Disneyland. (S3*) Bob had long hair in 1955. (S2) So if Bob visited Disneyland in 1955, he didn’t get in...

11 (1) Making Arguments Explicit
Examples (S1) The killer had a 1-inch scar above his left eye (S2) There are 5 people who had a motive for committing the murder (S3) Smith had a motive to commit the murder (S4) Smith has a 1-inch scar above his left eye (S5) So Smith is a suspect

12 (1) Making Arguments Explicit
Examples Let’s try that again: (*S6) Anyone who had a motive to commit the murder is a suspect (S3) Smith had a motive to commit the murder (S5) So Smith is a suspect (S1) The killer had a 1-inch scar above his left eye (S4) Smith has a 1-inch scar above his left eye (*S7) Therefore Smith is the killer

13 (2) Validity & Soundness
Could we convict Smith on the basis of this argument? If not, why not? How do we answer such questions? Features of a good argument (n.b. technical terms!): Validity: an argument is valid if and only if when the premises are true, the conclusion is true. Soundness: an argument is sound if and only if it is valid & its premises are true.

14 Criticising an Argument
(2) Validity & Soundness Criticising an Argument Thus, to show an argument is faulty, you can either show that: It is invalid: the conclusion does not follow from the premises. It is unsound: one or more of its premises is false. (Notice that if an argument is invalid, then it is automatically unsound.)

15 (2) Validity & Soundness
Valid & Sound? How should you decide whether an argument is valid and/or sound? Validity: Consider the form or pattern of the argument Always valid - ‘syllogisms’, Always invalid - ‘fallacies’ Think of a counter-example (Section 3) Soundness: Try to think of a counter-example (Section 3)

16 Valid ‘Patterns’ of Inference
(2) Validity & Soundness Valid ‘Patterns’ of Inference Check whether an argument is valid by looking at its form: (S1) If John paid the electricity bill today, then we do not have enough money to pay the gas bill. (S2) John paid the electricity bill today. (S3) So we do not have enough money to pay the gas bill.

17 (2) Validity & Soundness
Modus Ponens The form of that argument... If X, then Y X So Y. Arguments taking the form ‘modus ponens’ are syllogisms - they are always valid. (n.b. this does not mean they are always sound!)

18 (2) Validity & Soundness
Example (S1) If John paid the electricity bill today, then he would have looked miserable when you saw him. (S2) John did not look miserable when you saw him. (S3) So John did not pay the electricity bill today.

19 (2) Validity & Soundness
Modus Tollens The form of that argument... If X, then Y not-Y So not-X

20 (2) Validity & Soundness
Example (S1) If John paid the electricity bill today, then we do not have enough money to pay the gas bill. (S2) If we do not have enough money to pay the gas bill, Lisa will be angry. (S3) So if John paid the electricity bill today, then Lisa will be angry.

21 Hypothetical Syllogism
(2) Validity & Soundness Hypothetical Syllogism The form of that argument... If X, then Y If Y, then Z So if X, then Z

22 (2) Validity & Soundness
Conditionals Modus Ponens Modus Tollens Hypothetical Syllogism These are all valid patterns of inference for reasoning with conditional or hypothetical sentences (i.e. sentences of the form ‘If X, then Y’)

23 (2) Validity & Soundness
Other Valid Patterns Disjunctive Syllogism X or Y (a ‘disjunction’, X & Y are ‘disjuncts’) not-X So Y Example: (S1) Either we must pay our electricity bill or we must go bankrupt (S2) We must not go bankrupt (S3) So we must pay our electricity bill

24 (2) Validity & Soundness
Invalid Patterns ‘Affirming the Consequent’ Conditional sentence: ‘If X, then Y’ If X = the ‘antecedent’ then Y = the ‘consequent’ If X, then Y Y So X

25 Affirming the Consequent
(2) Validity & Soundness Affirming the Consequent (S1) If he is a thief, then he would look uncomfortable. (S2) He looks uncomfortable. (S3) So he is a thief. Problem: There are other antecedents (‘reasons why’) for the consequent ‘he would look uncomfortable’.

26 Denying the Antecedent
(2) Validity & Soundness Denying the Antecedent If X, then Y Not-X So not-Y (S1) If he says he posted the letter, then he posted it. (S2) But he hasn’t said he posted the letter. (S3) So he hasn’t posted the letter. Problem: The argument only tells you what will happen if he says he posted the letter. He didn’t say he posted it, so the argument cannot tell us anything.

27 Valid & Invalid Patterns
(2) Validity & Soundness Valid & Invalid Patterns Valid: Modus Ponens Modus Tollens Hypothetical Syllogism Disjunctive Syllogism (and many others...) Invalid: Affirming the Consequent Denying the Antecedent

28 (3) Counter-examples A counter-example = a case where a premise appears false (argument unsound) or the conclusion does not follow (argument invalid). (P1) All Muslims are extremists. (P2) All extremists are terrorists. (C) Therefore all Muslims are terrorists. Counter-examples for Soundness A Muslim who isn’t an extremist An Extremist who isn’t a terrorist

29 (3) Counter-examples C-Es for Soundness If you eat chocolate every week, then you are obese. Beth Ditto eats chocolate every week. So Beth Ditto is obese. Counter-example for soundness = I eat chocolate once a week, I am not obese.

30 C-Es for Validity (3) Counter-examples
Try to come up with an argument of the same form, but which is obviously mistaken. Example: (S1) If the ambassador were unhappy with her position in Portugal, she would be willing to take the position in Ireland. (S2) The ambassador was willing to take the position in Ireland. (S3) So the ambassador must have been unhappy with her position in Portugal.

31 C-Es for Validity (3) Counter-examples The form of that argument...
If X, then Y Y So X Recognise this pattern? A counter-example: (S1) If Julia Roberts is a lawyer, then she has been to university. (S2) Julia Roberts has been to university. (S3) So Julia Roberts is a lawyer. Mnemonic: ‘If that argument were valid, it could be used to show that... [insert absurd or false claim]’

32 Counter-examples For soundness:
Think of a case that shows a premise is false. For validity: Show how the same form or pattern of reasoning, when applied to different claims, leads to an obviously false conclusion.

33 A word about ‘Induction’
Not every good or strong argument is valid. Valid arguments always involve deductive reasoning - we deduce a conclusion from a set of premises. But there is also inductive reasoning, where a conclusion is supported by (for example) a large number of observations.

34 Induction (3) Counter-examples An example of induction:
Every zebra we have ever observed has black and white stripes. So all zebras have black and white stripes. Induction is ampliative, unlike deduction Goes beyond or adds to the information in the premises/observations Strictly invalid, thus it carries no guarantee of the truth of its conclusion Arguments can still be inductively strong

35 What You Have Learned Today
How to Make Arguments Explicit Identify premises & conclusions, eliminate extraneous material, add implicit premises/conclusions/sub-conclusions Validity and Soundness Validity: when the premises are true, the conclusion must be true. Soundness: it is valid, and the premises are true. Counter-examples For soundness: a case where a premise is false. For Validity: an argument of the same form with a false conclusion.


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