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Arguments and Fallacies

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Presentation on theme: "Arguments and Fallacies"— Presentation transcript:

1 Arguments and Fallacies
Assumptions Logical Rule(s) Conclusion(s) Validity Soundness Fallacy

2 An argument isn't just contradiction.
 It can be.      No it can't. An argument is a connected series of statements intended to establish a proposition. No it isn't.      Yes it is! It's not just contradiction. Look, if I argue with you, I must take up a contrary position.      Yes, but that's not just saying 'No it isn't.' Yes it is!      No it isn't! Argument is an intellectual process. Contradiction is just the automatic gainsaying of any statement the other person makes.      It is.

3 The term "fallacy" does not mean false statement.
Critical Thinking Skills for Identifying Logical Fallacies in Everyday Reasoning The term "fallacy" does not mean false statement. It means faulty reasoning. So it is possible for an argument to contain all true statements and still be fallacious.

4 Some Common Informal Fallacies
Ad Hominem Argument Slippery Slope Argument Fallacy of Appeal to Authority False Cause Fallacy Begging the Question Fallacy of Composition/Fallacy of Division Fallacy of Ambiguity Appeal to the People (Argumentum ad Populum) The Many/Any Fallacy The Virtuality Fallacy

5 Ad Hominem From Latin, "Ad Hominem" means "against the man" or "against the person." fallacies in which a claim or argument is rejected on the basis of some irrelevant fact about the person making the claim. Typically Person A makes claim X. Person B makes an attack on person A. Therefore A's claim is false. Fallacy: circumstances or actions of a person do not (in most cases) have a bearing on the truth or falsity of the claim.

6 Example of Ad Hominem Jessie: "I believe that file sharing of music should be stopped." Ben: “You say that, but I noticed that you downloaded an MP3 today." Jessie: "What about the arguments I gave to support my position?" Ben: "Those don't count. You don’t practice what you preach, so I can't believe what you say."

7 Another example of Ad Hominem?
Clinton is an adulterer He lacks moral “character” ____________________ So, he will not support good policy, and he will make a bad president

8 Slippery Slope Fallacy (Camels nose)
If you allow a camel to poke his nose into the tent, soon the whole camel will follow. Something is wrong because it is next to something that is wrong. Or, it is wrong because it could slide towards something that is wrong. Momentum: event A will initiate a process which will lead inevitably to event B. Domino Theory Gateway drugs, gun control will lead to gun confiscation Induction: like mathematical induction (almost) If Blacksburg is far from New York, then 1 mile closer is still far. So 2 miles closer is far, and 3, and 4…. Therefore n miles is far If 5 is a lot then 4 is a lot. If 4 is a lot, then 3 is a lot… If 1 is a lot then 0 is lot. If 0 is a lot then….

9 Affirming The Consequent
logic reversal. A correct statement of the form "if P then Q" gets turned into "Q therefore P". For example, "All people whose surname begins with Mac are of Scottish ancestry. Dougal is of Scottish ancestry. Therefore his surname begins with Mac." But actually his name is Campbell. Another example: "If the earth orbits the sun, then the nearer stars will show an apparent annual shift in position relative to more distant stars (stellar parallax). Observations show conclusively that this parallax shift does occur. This proves that the earth orbits the sun." In reality, it proves that Q [the parallax] is consistent with P [orbiting the sun]. But it might also be consistent with some other theory. (Other theories did exist. They are now dead, because although they were consistent with a few facts, they were not consistent with all the facts.)

10 False Cause (Correlation vs. Cause)
Assuming that because two things happened, the first one caused the second one. (Sequence is not causation.) "Every time my brother Bill accompanies me to Fenway Park, the Red Sox are sure to lose." The bigger a child's shoe size, the better the child's handwriting. When sales of hot chocolate go up, street crime drops.

11 Begging The Question Assuming The Answer, Tautology
Reasoning in a circle. The thing to be proved is used as one of your assumptions. For example: "We must have a death penalty to discourage violent crime". (This assumes it discourages crime.) "The stock market fell because of a technical adjustment." (But an "adjustment" IS just a stock market fall).

12 Fallacy Of Composition
Assuming that a whole has the same simplicity as its constituent parts. "A car makes less pollution than a bus. Therefore, cars are less of a pollution problem than buses." Another example: "Atoms are colorless. Cats are made of atoms, so cats are colorless." Assuming that what is true of the whole is true of each constituent part. For example, human beings are made of atoms, and human beings are conscious, so atoms must be conscious. Fallacy Of Division

13 Non Sequitur Something that just does not follow.
For example, "Tens of thousands of Americans have seen lights in the night sky which they could not identify. The existence of life on other planets is fast becoming certainty!" "Bill lives in a large building, so his apartment must be large."

14 Statistics President Dwight Eisenhower expressed astonishment and alarm on discovering that fully half of all Americans had below average intelligence. Most 3rd graders read at or below 3rd grade level “Regression to the mean“ = things tend to go back to normal. E.g. cures.

15 Straw Man Fallacy Of Extension
Attacking an exaggerated or caricatured version of your opponent's position. “Evolution means a dog giving birth to a cat." "Senator Jones says that we should not fund the attack submarine program. I can't understand why he wants to leave us defenseless like that."

16 Fallacy of Appeal to Authority
Fallacy of Ambiguity Appeal to the People (Argumentum ad Populum) The Virtuality Fallacy

17 If (A) you’re in Dr. D’s class then (B) you’re happy
Modus Ponens A  B You’re in class (A) Therefore You’re happy (B) Contrapositive ~B  ~A (Modus Tollens) You’re not happy (~B) Therefore you’re not in class (~A) Affirming the consequence (Fallacy) You’re happy (B) Therefore you’re in class (A)

18 DeMorgan's Law used to distribute a negative
to a conjunction or disjunction. ~(C and P): "It is not true that Dan is nice and smart." ~C or ~P: “Dan is not nice or he is not smart." ~(P or Q): "It is not true that class is boring or drinking is interesting.“ ~P and ~Q: “Class is not boring and drinking is not interesting."

19 Arguments: Valid, Sound, Strong, Weak
Invalid Unsound Sound Inductive Fallacious Weak Arguments Strong Arguments Weak Arguments

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