Logic arguments and_fallacies

Logic: Arguments and Fallacies

The Nature of Argument

Arguments

Logic (def.): The science that evaluates arguments. Argument (def.): A group of statements, a group of

which serve (the premises) to support, imply, or provide evidence for another statement. (the conclusion).

Premises (def.): Set forth the reasons for the conclusion.

Logic--The primary task: To distinguish between good arguments and bad arguments. A good argument is one in which the premises support the conclusion.

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Socrates is a man. PremiseAll men are mortal. Premise

Therefore, Socrates is mortal. Conclusion

Types of Arguments

1. Deductive Argument (def.): When an argument has thepurport of proving its conclusion necessarily from the premises.

An argument is deductive if its purport is that it is impossible that its premises be true and its conclusion false.

2. Inductive Argument (def.): When an argument has the purport of showing its conclusion to be likely or probable given the premises.

An argument is inductive if its purport is merely that it isimprobable that its premises be true and its conclusion false.

Arguments: How to Identify

1. Properties of Deductive Arguments:The conclusion follows, or thought to follow

necessarily from the premises. In drawing its conclusion, the argument employs such

words as “necessarily,” “certainly,” or “absolutely,” it is usually best regarded as deductive.

2. Properties of Inductive ArgumentsThe words such as “probably,” “likely,” or “plausibly” are employed.

Argument Types

Deductive Arguments (types): 1. Categorical Syllogisms (e.g., All N’s are x; S is an N; S is an x).2. Hypothetical Syllogisms (e.g., If P then Q; P, therefore Q).3. Disjunctive Syllogisms (e.g., Either C or T; Not S, therefore T).

Inductive Arguments (types): 4. Predictions about the future. 5. Arguments from analogy. 6. Inductive generalizations.7. Many arguments from authority. 8. Arguments based on signs, and causal inferences.

Validity and Soundness

Validity: A deductive argument is either valid or invalid. A deductive argument is valid if the conclusion follows

necessarily from the premises: If it is necessarily the case that if the premises were to be true, then the conclusion must true (whether it is in fact true or not).

If there is any possibility that the all the premises could be true and the conclusion false, the argument is invalid.

NOTE: The truth of the premises is not required for validity.

Strength and CogencyStrong Inductive Argument: If on the basis of the assumption that its

premises are true, its conclusion probably is true; otherwise, it is weak. Strength admits of degrees.

A : Ninety percent of the mice in Australia have been examined and found to be white; therefore probably all of the mice in Australia are white.

B: Ninety nine percent of the mice in Australia have been examined and found to be white; therefore, probably all of the mice in Australia are white.

Both A. and B. are Strong, but B is even stronger than A.

Cogent argument: is an inductive argument that is strong and has all true premises. The conclusion will probably also be true.

Fallacies

A fallacy is a defect in an argument other than its having false premises.

Types:Informal Fallacy: A fallacy that requires an analysis

of the content of the argument and not just an inspection of its form.

Formal Fallacy: A fallacy that may be identified by a mere inspection of the form of the argument.

Fallacies of Relevance

The appeal to force (argumentum and baculum) occurs when the arguer, instead of providing genuine evidence for a conclusion, provides some sort of threat of harm to the listener or reader if the conclusion is not accepted. E.g., Either you can pay me you the ten thousand you owe me, or you can pay your dentist.

The appeal to pity (argumentum ad misericordiam) occurs when the arguer, instead ofproviding genuine evidence for a conclusion, attempts to get the conclusion accepted by evoking pity from the listener or reader. E.g., Our company is on the rocks, financially, if you sue us, we will go out of business, and our children will not be able to go to college.

Fallacies of Relevance (cont.)

The appeal to the people (argumentum ad populum) when the arguer tries to get the conclusion accepted by playing upon the listener’s desire to be loved, esteemed, admired, valued, recognized, or accepted. E.g., Everybody knows that Smith cannot win, so you should vote for Connor in the election.

Argument against the person (argumentum ad hominem) when one arguer directs attention to the person of a second arguer and not to the second arguer’s argument or position. E.g., You graduated with a PhD from NYU, I’m surprised that you don’t believe that humans are responsible for climate change!

Fallacies of Relevance (cont.)

Fallacy of accident: When a general rule is wrongly or unjustifiably applied to a specific case. E.g., Dogs have four legs; Fido just had one of his legs amputated; so Fido is not a dog any more.

Straw man fallacy: When an arguer distorts a certain argument or position for in order to attack it, refutes the distorted argument or position, and then concludes that the real argument or position has been refuted. E.g., The Bible should not be taught in schools, because that’s what religious zealots want,

Fallacies of Relevance (cont.)

Fallacy of Missing the Point (ignoratio elenchi) occurs when the premises of an argument lead, or seem to lead, to one conclusion and then a completely different conclusion is drawn. E.g., Abuse of the welfare system is rampant nowadays. Our only alternative is to abolish the system altogether.

Red Herring Fallacy is similar to the fallacy of missing the point. It occurs when an arguer diverts the attention of the reader or listener by going off on extraneous issues and points but ends by assuming that some conclusion relevant to the point at hand has been established. E.g., Our twelve year old boys are failing in mathematics. I just discovered that they were looking at pornographic websites last night. So you need to learn how to keep tabs on their Internet use!

Fallacies of Weak Induction

Unqualified Authority (argumentum ad verecundiam): When an arguer cites the testimony or belief of an authority who is not necessarily reliable or who is not an expert in the subject at hand. E.g., He has a PhD in Physics, that makes him a doctor, so we should ask him if I have Swine Flu!

Appeal to ignorance (argumentum ad ignorantiam): When the premises state that nothing is known with certainty about a certain subject, and the conclusion states something definite about that subject. E.g., People have been trying for centuries to disprove the claims of astrology. But no one has ever succeeded. So astrology is just nonsense.

Fallacies of Weak Induction (cont.)

Hasty Generalization (converse accident): When a conclusion is drawn about all the members of a group or population from premises about some sample of the group that is not representative. E.g., When I wore this copper bracelet, I broke out into a rash. I must be allergic to copper.

False Cause: When the link between premises and conclusion in an argument depends on the supposition of some causal connection that does not in fact exist. E.g.,: The clock chimed six times, and then the sun came up; the sun would not have come up without the clock chiming six times.

Fallacies of Weak Induction (cont.)

Slippery Slope: When the conclusion of an argument depends on the claim that a certain event or situation will ultimately lead to an undesirable consequence, without justification. E.g., If we start letting newspapers publish their news online, then one of these days there will be no more newspapers and the news industry will become obsolete.

Weak analogy: When the analogy between two things is not strong enough to support the conclusion; sometimes it is a lack of causal connections between the attributes. E.g., A has attributes a, b, c, d, and z; B has attributes a, b, c, and d; So B probably has z.

Fallacies of Weak Induction (cont.)

Begging the Question (petitio principii) When the arguer uses some trick or device to hide the fact that a premise may not be true. E.g., If it weren’t for Global Warming, we wouldn’t be suffering from four weeks of 85 degree-plus weather in September.

Complex Question When an apparently single question is asked that really involves two or more questions, answerable by single answer. E.g., Have you stopped drowning kittens for fun?

Fallacies of Ambiguity

Fallacy of Suppressed Evidence Consists in passing off what are at best half-truths as if they were the whole truth and using them as premises in an argument. E.g., You ought to learn to play golf, because executive assistants make excellent money and acquire great perks.

Fallacy of Equivocation When the inference in an argument depends on the fact that a word or phrase is used in two or more different senses. E.g., Banks have lots of money in them; the sides of rivers are banks; therefore, the sides of rivers have lots of money in them.

Fallacies of Ambiguity (cont.)

Fallacy of Amphiboly: When an arguer, beginning with some statement that is ambiguous owing to its syntactical structure, proceeds to interpret it in a way in which it was not intended and to draw a conclusion based on this faulty interpretation. E.g., Last night Scott cuddled his dog in his pajamas. Why Scott put the dog in his pajamas I’ll never know.

A False Dichotomy A pair of alternatives, presented as if it were a dichotomy when it is not in fact a dichotomy. E.g., You can ride the bus, or you can take your lunch.

Fallacies of Analogy

Fallacy of Composition: When the inference in an argument depends on the erroneous transference of a characteristic from the parts of something to the whole. E.g., Hydrogen and Oxygen are gases; therefore, H2O is a gas.

Fallacy of Division When the inference in an argument depends on the erroneous transference of a characteristic from a whole to some one or more of its parts. E.g., Salt is a non-poisonous compound. Therefore, it’s component elements, sodium and chlorine, are non-poisonous [FALSE]

Formal Fallacies

Fallacy of Affirming the Consequent: Consists of one conditional premise, a second premise that asserts the consequent of the conditional, and a conclusion that asserts the antecedent.

For example:a. If Napoleon was killed in a plane crash, then he is dead.b. Napoleon is dead.c. Therefore, Napoleon was killed in a crash.

This fallacy has the form:If P then QQ, so P.

Formal Fallacies (cont.)

The Fallacy of Denying the Antecedent: Consists in a conditional premise, a second premise that denies the antecedent of the conditional, and a conclusion that denies the consequent:

a. If Napoleon was killed in a plane crash, then Napoleon is dead.b. Napoleon was not killed in a plane crash.c. Therefore, Napoleon is not dead.

This fallacy has the formIf P then Q.Not Q, so not-P.