2Outline 1 – Arguments: valid vs sound 2. Conditionals 3. Common Forms of Bad Arguments
3Outline 1 – Arguments: valid vs sound 2. Conditionals 3. Common Forms of Bad Arguments
4Arguments Argument: a sequence of sentences where one (the conclusion) is meant to follow from or be supported by the others (the premises). P1 All men are mortal. P2 Socrates is a man. C. Socrates is mortal. Premise 1 Premise 2 Conclusion Arguments can be: 1.Valid / Invalid 2.Sound / Unsound
5 Valid Arguments Valid Arg.: -The conclusion logically follows from the premises - An argument where it is impossible for the premises to be true and the conclusion false. P1 All cats are reptile. P2 Bug Bunny is a cat. CC Bugs Bunny is a reptile. P1 All men are mortal. P2 Socrates is a man. CC Socrates is mortal. VALIDITY = LOGICAL FORM
6 Valid Arguments Attention! -Just because an argument is valid, it does not mean that its conclusion is true - Just because an argument is invalid, it does not mean that its conclusion is false. P1 All cats are reptiles. P2 Bugs Bunny is a cat. CC Bugs Bunny is a reptile. P1 If Bugs Bunny has big ears, then he is a reptile P2 Bugs Bunny is a reptile. CC Bugs Bunny has big ears. But if the premises are true and the argument is valid, then the conclusion is always true
7 Valid Arguments : Exercise I Your high idle is caused either by a problem with the transmission, or by too little oil, or both. You have too little oil in your car. Therefore, your transmission is fine. II If the moon is made of green cheese, then cows jump over it. The moon is made of green cheese. Therefore, cows jump over the moon. III Either Colonel Mustard or Miss Scarlet is the culprit. Miss Scarlet is not the culprit. Hence, Colonel Mustard is the culprit. IV All engineers enjoy ballet. Therefore, some males enjoy ballet.
9 Valid vs Sound An argument which is valid but not sound: P1. If the moon is made of green cheese, then cows jump over it. P2. The moon is made of green cheese. CC Therefore, cows jump over the moon. Sound Arg: P1 Prof. Le Bihan’s baby is a girl or a boy. P2 Prof. Le Bihan’s baby is not a boy. CC. Prof. Le Bihan’s baby is a girl.
10 Conclusion: Good and Bad Arguments Good arguments are SOUND, ie are valid and possess true premises – They support the conclusion Bad arguments are either: -Invalid : the logical form is incorrect, or -Unsound: the premises are false These are the two ways in which you can criticize an argument from a philosopher ! To Remember: distinction valid / sound arguments
11 Arguments: Exercise! I.If Socrates is a man, then Socrates is mortal. Socrates is a man. So, Socrates is mortal. II.If Socrates is a horse, then Socrates is mortal. Socrates is a horse. So, Socrates is mortal. III.If Socrates is a horse, then Socrates has four legs. Socrates is a horse. So, Socrates has four legs. IV.If Socrates is a horse, then Socrates has four legs. Socrates doesn’t have four legs. So, Socrates is not a horse.
12 Arguments: Exercise! I.If Socrates is a man, then he’s a mammal. Socrates is not a mammal. So Socrates is not a man. II.If Socrates is a horse, then he’s warm-blooded. Socrates is warm- blooded. So Socrates is a horse. III.If Socrates was a philosopher then he wasn’t a historian. Socrates wasn’t a historian. So, Socrates was a philosopher.
13Outline 1 – Arguments: valid vs sound 2. Conditionals 3. Common Forms of Bad Arguments
14Conditionals Conditional : IF P THEN Q / P Q Necessary and Sufficient conditions: IF P THEN Q P Sufficient Q Necessary If you are pregnant then you put on weight
15 Conditionals: Exercise! – to be over 16 / to be legally driving – to be human / to be an animal – to be alive / to be sick – to be alive / not to be dead – to be rectangular / to be square – to be possible / to be real – to do your homework / to get a good grade
16Conditionals Attention: IF P THEN Q is NOT equivalent to IF Q then P Attention: IF P THEN Q is NOT equivalent to IF Q then P If you are pregnant, you put on weight Is NOT equivalent to: If you put on weight then you are pregnant
17Contrapositions Contraposition: IF P THEN Q is equivalent to IF not Q then not P Contraposition: IF P THEN Q is equivalent to IF not Q then not P If you are pregnant, you put on weight IS equivalent to: If you DON’T put on weight then you are NOT pregnant
18 Conclusion on Conditionals To Remember: 1.What a conditional is 2.The distinction between necessary and sufficient conditions 3.The notion of contraposition To Remember: 1.What a conditional is 2.The distinction between necessary and sufficient conditions 3.The notion of contraposition
19 Conditionals: Exercise! –You don’t drink? Oh, then you must be pregnant! – You’re pregnant? Oh, poor girl, you’re stuck with carrot juice then! – No pregnant women drink alcohol – You are drinking tonight? I see, you are not pregnant yet. – Wait, you are not pregnant, right? So, you are drinking tonight! – If Justin is pregnant then he does not drink
20Outline 1 – Arguments: valid vs sound 2. Conditionals 3. Common Forms of Bad Arguments
21 Using a conditional the wrong way Two ways to use a conditional the wrong way: Affirming the consequent/ Denying the antecedent Two ways to use a conditional the wrong way: Affirming the consequent/ Denying the antecedent Affirming the antecedent P you are pregnant, Hence Q then you gain weight Affirming the consequent Q you gain weight, Hence P then you are pregnant Denying the antecedent Non Q you are not pregnant, Non P then you don’t gain weight Denying the consequent Non P you don’t gain weight, Non Q then you are not pregnant IF P THEN Q If you are pregnant, then you gain weight IF P THEN Q If you are pregnant, then you gain weight
22 Circular Arguments Circular arguments assume what they want to prove. The police did not beat the suspect because beating suspects in not something that police ever do. We know that God exists, because it says so in the Bible. And we can trust the Bible on this matter because it’s the Word of God, and so it must be correct. Darwin's account of evolution is just a theory. A theory is an unproven hypothesis. So, there is no compelling reason to believe Darwin's theory.
23 Consequential Fallacy Consequential fallacy: confuse the consequences of holding a belief with evidence for that belief. Darwin’s theory is false because if it were true, there would be no morality.
24Equivocation Equivocation: to use an ambiguous term in different ways in an argument. Nature is governed by ﬁxed and unchangeable laws. But every law is the work of some legislator. Therefore, there is some legislator responsible for the governing of Nature.
25 Appeal to Consensus Appeal to Consensus: one appeals to consensus to establish a claim. Most people believe that McDonals is the best restaurant in the world. McDonalds is the best restaurant in the world.
26 Slippery Slope Slippery slope: incorrectly reasons that the arbitrariness of marking a distinction along some continuum shows that no distinction is possible. There is no agreed upon number of hair that qualifies someone as bald. Therefore, there is no difference between being bald or hairy. There is no agreed upon age that qualifies a fetus as a person. To kill an innocent person is murder. Therefore, all abortions are murders, independently of the age of the fetus.
27 Misleading Vividness Misleading vividness: particularly vivid information is weighted more than other information in coming to a conclusion. Elections in France 2007
28 Genetic Fallacy Genetic fallacy: taking the source of a claim as evidence for or against the claim. Tom Cruise said that there is no such thing as a chemical imbalance of the body. Tom Cruise is crazy. Therefore, there is such thing as a chemical imbalance of the body.
29 Straw Man Straw man: misrepesenting someone’s position, arguing against it, and supposing that the actual position is defeated.
30 Straw Man Nigel: I believe that some kinds of sexual lifestyles are morally wrong. Basil: So, you’re saying it’s OK for rednecks to beat up gay people? Nigel: No, I’m not saying that at all. All people in our society should be protected from having unwanted violence inflicted upon them. I’m just saying I think their chosen lifestyles are immoral.
31 Straw Man Basil: What makes you think it’s OK for you to force your morality on everyone else? Nigel: I haven’t said anything about forcing my morality on anybody. All I did was give my opinion about a certain moral issue. I didn’t use any force or even the threat of force to coerce others to agree with me.
32 Straw Man Basil: But you are saying that you don’t think gay people should have the same rights as straight people, right? Nigel: No. I think all people in a democratic society should have the same rights. That means that people should have the right to pursue lifestyles that others think are immoral. I haven’t said anything about depriving people of their rights or inflicting violence upon them. I’m only giving my opinion about the morality of their behavior. Bebee (2003) ``Good and Bad Arguments.’’
33Feelings Feelings: a person argues for a position by indicating that they feel a certain way. Problem: No one cares how you feel. Solution: Use good arguments! More important problem: There is no warrant the think that your feelings about something are correlated to the truth or falsity of a proposition.
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