What is a fallacy? Fallacies of Relevance Defective Induction Fallacies of Presumption Ambiguity Summary. Logic 2: Fallacies Jan.

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1 Logic 2: Fallacies Jan. 17, 2014

2 Overview I What is a fallacy? Definition Formal and Informal Fallacies Fallacies of Relevance Appeal to Emotion Appeal to Pity Appeal to Force Argument Against the Person Irrelevant conclusion Summary Defective Induction Fallacy of Ignorance Appeal to Inappropriate Authority False Cause Hasty Generalization

3 Overview II Summary Fallacies of Presumption Fallacy of Accident Complex Question Begging the Question Summary Ambiguity Equivocation Amphiboly Fallacy of Composition Fallacy of Division Summary Summary

4 Definition Formal and Informal Fallacies When an argument goes wrong... An argument can go wrong in many ways. Fallacy: some (or all) of the premises are obviously false the conclusion does not follow it has the wrong form (invalid argument) inductive arguments can also go wrong in many ways But sometimes these mistakes are very subtle and hard to notice. When the premises of an argument appear to support the conclusion, but do not in fact do so, we say that the argument is fallacious. By fallacy we mean typical errors that often occur, and are often deceiving.

5 Definition Formal and Informal Fallacies Formal and Informal Fallacies Note: all we will talk about here are informal fallacies, that is, cases where the argument goes wrong in some obvious way, and we do not need formal logic to discover the fallacy. (This means that there are also formal fallacies, and we will treat them later.)

6 Appeal to Emotion Appeal to Pity Appeal to Force Argument Against the Person Irrelevant conclusion Summary As all clear-thinking residents of our fine state have already realized, the Governor s plan for financing public education is nothing but the bloody-fanged wolf of socialism cleverly disguised in the harmless sheep s clothing of concern for children. Therefore, the Governor s plan is bad public policy. What s wrong with this? Does the conclusion follow? Why? Appeal to emotion ad populum The argument relies on emotion rather than on reason: In place of evidence, it uses expressive language to excite enthusiasm for or against some cause. But the occurrence of this enthusiasm has nothing to do with the truth of the conclusion.

7 Appeal to Emotion Appeal to Pity Appeal to Force Argument Against the Person Irrelevant conclusion Summary I am a single parent, solely responsible for the financial support of my children. If you give me this traffic ticket, I will lose my license and be unable to drive to work. If I cannot work, my children and I will become homeless and may starve to death. Therefore, you should not give me this traffic ticket. What s wrong now? Can the premises be true and the conclusion false? Appeal to pity ad misericordiam Again, the argument relies on emotion rather than on reason: it tries to convince you by pointing out the unfortunate consequences that will otherwise follow, for which we would then feel sorry. The truth of the premises, again, has nothing to do with the truth of the conclusion.

8 Appeal to Emotion Appeal to Pity Appeal to Force Argument Against the Person Irrelevant conclusion Summary If you do not agree with my opinions, you will receive a grade of F for this course. I believe that Aristotle was the greatest philosopher of all times. Therefore, Aristotle was the greatest philosopher of all times. Can the premises be true and the conclusion false? What if the conclusion is also true? Appeal to force ad baculum Someone in a position of power threatens to bring about unfortunate consequences for anyone who dares to disagree with his/her proposition. While this might be an effective way to get you to agree (or at least to pretend to agree) with my position, it offers no reasons for believing it to be true.

9 Appeal to Emotion Appeal to Pity Appeal to Force Argument Against the Person Irrelevant conclusion Summary Before he died, poet Allen Ginsberg argued in favor of legalizing pornography. But Ginsberg s arguments are nothing but trash: he smoked marijuana and was an advocate of the drug culture. It is one of the most common fallacies; very often occurs, and we don t even notice it. Argument against the person ad hominem Instead of arguing against someone s opinion, the argument attacks the person who holds that opinion by showing him as disreputable in some way. So it is saying that the opinion must be false because of the person who believes it to be true. (But again, personality is irrelevant to truth!)

10 Appeal to Emotion Appeal to Pity Appeal to Force Argument Against the Person Irrelevant conclusion Summary All children should have ample attention from their parents. Parents who work full-time cannot give ample attention to their children. Therefore, mothers should not work full-time. Do we have an argument here? Does the conclusion follow? Why? Irrelevant conclusion ignoratio elenchi The speaker tries to establish the truth of a proposition by offering an argument that actually provides support for an entirely different conclusion. It can often distract the audience, and we don t notice that the conclusion just misses the point.

11 Appeal to Emotion Appeal to Pity Appeal to Force Argument Against the Person Irrelevant conclusion Summary 2 especially common cases of irrelevant conclusion: Red herring: A deliberate attempt to change the subject or divert the argument from the real question at issue to some side-point. I should not pay a fine for reckless driving. There are many other people on the street who are dangerous criminals and rapists, and the police should be chasing them, not harassing a decent tax-paying citizen like me.

12 Appeal to Emotion Appeal to Pity Appeal to Force Argument Against the Person Irrelevant conclusion Summary 2 especially common cases of irrelevant conclusion: Straw man: An attempt to establish a conclusion by overstating, exaggerating, or over-simplifying the arguments of the opposing side. I say: New York should increase funding to unemployed single mothers during the first year after childbirth because they need sufficient money to provide medical care for their newborn children. The other side: My opponent believes that some parasites who don t work should get a free ride from the tax money of hard-working honest citizens. I ll show you why he s wrong...

13 Appeal to Emotion Appeal to Pity Appeal to Force Argument Against the Person Irrelevant conclusion Summary Summary: Fallacies of Relevance Fallacies of relevance are bald mistakes; they might better be called fallacies of irrelevance: they point to the absence of any real connection between the premises and the conclusion of the argument. Since there is no connection, the premises cannot possibly establish the truth of the conclusion. But the premises are usually psychologically relevant: they have some emotional impact on the readers.

14 Appeal to Emotion Appeal to Pity Appeal to Force Argument Against the Person Irrelevant conclusion Summary Summary: Fallacies of Relevance 1 Appeal to emotion 2 Appeal to pity 3 Appeal to force 4 Argument against the person 5 Irrelevant conclusion

15 Fallacy of Ignorance Appeal to Inappropriate Authority False Cause Hasty Generalization Summary No one has conclusively proven that there is no intelligent life on the moons of Jupiter. Therefore, there is intelligent life on the moons of Jupiter. Again, the conclusion does not follow; the opposite has not been proven either. Notice that the procedure is sometimes used rightly! E.g., in court. Fallacy of ignorance ad ignorantiam The speaker asserts something only because the opposite has not been proven. But from this, the conclusion does not follow: the opposite might still be true, just no proof found (yet), or maybe it cannot be proven for some reason.

16 Fallacy of Ignorance Appeal to Inappropriate Authority False Cause Hasty Generalization Summary The former Governor believes that aliens have landed in the Arizona desert, so aliens must have landed in the Arizona desert. Why does it matter what the Governor believes? Appeal to inappropriate authority ad verecundiam Tries to establish a conclusion by appeal to an improper authority, such as a famous person or a source that may not be reliable for some reason (e.g., biased authority). Since we know the person, or believe that he has some sort of knowledge, we assume that he has knowledge about things outside his expertise too.

17 Fallacy of Ignorance Appeal to Inappropriate Authority False Cause Hasty Generalization Summary A black cat crossed my path at noon. An hour later, my mother had a heart-attack. So the black cat must have caused the bad luck. The death penalty in the US has given us the highest crime rate and greatest number of prisoners per 100,000 population in the industrialized world. From the fact that one event was before another one, can we infer that the latter was caused by the former? False cause post hoc, ergo propter hoc A fallacy that occurs when the writer mistakenly assumes that, because the first event preceded the second event, it must mean the first event caused the later one. Sometimes it does, but sometimes it doesn t. But if it does, we have to establish it instead of just assuming.

18 Fallacy of Ignorance Appeal to Inappropriate Authority False Cause Hasty Generalization Summary Take my son, Martyn. He s been eating fish and chips his whole life, and he just had a cholesterol test, and his level is below the national average. What better proof could there be than a fryer s son? Is the sample a good sample? Did we examine enough / representative cases before drawing the conclusion? Hasty generalization dicto simpliciter Draws a general conclusion without examining all the relevant data. Of course, very often we can t examine everything; but still, the sample must be carefully selected, and large enough.

19 Fallacy of Ignorance Appeal to Inappropriate Authority False Cause Hasty Generalization Summary Summary: Fallacies of Defective Induction Unlike in the previous category (fallacies of relevance), in this case the premises are relevant to the conclusion. The problem is that they are inadequate: they are simply insufficient to establish the conclusion. 1 Fallacy of ignorance 2 Appeal to inappropriate authority 3 False cause 4 Hasty generalization

20 Fallacy of Accident Complex Question Begging the Question Summary Suppose that a friend in his right mind has deposited arms with me and asks for them when he is not in his right mind. Ought I to give the weapons back to him? Of course: everyone agrees that you should give back what you borrow. Is the general rule correct? How general is it? Fallacy of accident It occurs when one applies a general rule to a particular case when accidental circumstances render the general rule inapplicable. What is true in general might not be true absolutely universally and without qualification.

21 Fallacy of Accident Complex Question Begging the Question Summary With all of the hysteria, and phony science, could it be that man-made global warming is the greatest hoax ever perpetrated on the American people? What kind of question is it? What answer is the speaker expecting? Complex question fallacy A question is asked in a way that presupposes the truth of some proposition hidden within the question. The question is often rhetorical, no answer is genuinely asked. But if we analyze it, it might turn out that the questioner is presupposing something false.

22 Fallacy of Accident Complex Question Begging the Question Summary To allow every man unbounded freedom of speech, must always be, on the whole, advantageous to the state; for it is highly conducive to the interests of the community that each individual should enjoy a liberty, perfectly unlimited, of expressing his sentiments. Which are the premise and the conclusion? Do they really differ? Begging the question petitio principii Occurs when one assumes the truth of what one tries to prove. Can be very obvious, but not always! Also occurs in circular arguments, when I show that a is true because of b, b is true because of c, and c is true because of a.

23 Fallacy of Accident Complex Question Begging the Question Summary Summary: Fallacies of Presumption These are arguments that are based on some tacit (therefore unjustified) assumption. The assumption might even be true, but if it is unjustified, it cannot help to establish the conclusion. That the assumption is implicit might be either deliberate (when the author tries to hide some questionable claims), or just an oversight. It also might be the case that the author thinks the assumption is obvious, when in fact it is not. 1 Fallacy of accident 2 Complex question 3 Begging the question

24 Equivocation Amphiboly Fallacy of Composition Fallacy of Division Summary An elephant is an animal. Therefore, a small elephant is a small animal. Apples are fruits. My computer is an apple. Therefore, my computer is a fruit. What is funny in these arguments? Equivocation Occurs when a term or phrase is used in different senses in the premises and in the conclusion. It often occurs with relative terms ( small, tall, etc.), and those cases can be harder to notice.

25 Equivocation Amphiboly Fallacy of Composition Fallacy of Division Summary The tour guide said that standing in Greenwich Village, the Empire State Building can easily be seen. Thus, the Empire State Building is in Greenwich Village. What is the misleading point here? Amphiboly two in a lump Depends on an amphibolous statement, the meaning of which is indeterminate because of the way its words are combined. An amphibolous statement might be true in one interpretation and false in another; when it is used in the premise in one interpretation, and the conclusion is drawn based on the other one, that s a fallacy.

26 Equivocation Amphiboly Fallacy of Composition Fallacy of Division Summary Since a bus uses more gas than an automobile, therefore all buses use more gas than all automobiles. Since every part of a helicopter is lightweight, it follows that the whole helicopter is lightweight. Why does not the conclusion follow? Are there cases when it would follow? Fallacy of Composition An inference from the property of the parts to the property of the whole. Sometimes such inference is a valid one, but not always we need to check whether it works in the actual context.

27 Equivocation Amphiboly Fallacy of Composition Fallacy of Division Summary American Indians are disappearing. That man is an American Indian. Therefore, that man is disappearing. Many people have dogs. Afghan hounds are dogs. Therefore, many people have Afghan hounds. Why does not the conclusion follow? Are there cases when it does? Fallacy of Division The opposite of the fallacy of composition; here, it argues that what is true of a whole must also be true of its parts. Again, sometimes such an argument works, but we must be very careful with the form of the argument.

28 Equivocation Amphiboly Fallacy of Composition Fallacy of Division Summary Summary: Fallacies of Ambiguity The meaning of words of phrases may shift as a result of inattention, or deliberately within a course of an argument: a term may have one sense in a premise, but a different sense in the conclusion. If the inference depends on such changes in meaning, the argument is fallacious. 1 Equivocation 2 Amphiboly 3 Fallacy of composition 4 Fallacy of division

29 Summary: The ways an argument can go wrong Fallacies of relevance 1 Appeal to emotion 2 Appeal to pity 3 Appeal to force 4 Argument against the person 5 Irrelevant conclusion Fallacies of presumption 1 Fallacy of accident 2 Complex question 3 Begging the question Fallacies of defective induction 1 Fallacy of ignorance 2 Appeal to inappropriate authority 3 False cause 4 Hasty generalization Fallacies of ambiguity 1 Equivocation 2 Amphiboly 3 Fallacy of composition 4 Fallacy of division