Ch08 evaluating arguments

Ch 8Evaluating Arguments

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Once you have an argument summarized/standardized, you need to evaluate it to see if you are forced to accept the conclusion.

There are two main questions to ask when doing so: Is the argument a “good argument”? Are the premises acceptable?

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An argument is NOT good simply because it: “agrees with my views”

The attitude that only arguments that agree with your viewpoints are good is extremely close-minded.

“is a persuasive argument” People aren’t always smart and can be

persuaded by “eloquent speech” (and be confused by solid reasoning).

Hitler was more persuasive than Churchill, but that doesn’t mean that Hitler’s arguments were better. 3

“is well-written/spoken” Although it’s easier to tell whether an

argument is good if it is well written, being well written doesn’t make it good. Clarity, eloquence and organization can all occur in the presence of logical mistakes.

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It must, at the least, be either deductively sound (valid with true premises) or inductively cogent (strong with true premises).

It will also need to be clear… An argument isn’t good unless it is understandable.

…precise… One needs to avoid equivocation and use exact

language. …the premises need to be relevant…

Arguments with a lot of irrelevant material can’t be said to be good arguments.

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…consistent… Arguments that contain logical contradictions

commit the fallacy of inconsistency. …complete…

If an arguer ignores facts relevant to the conclusion at hand, we can’t say the argument is good (it doesn’t account for relevant objections).

…and fair. An argument can’t be good if it hastily dismissed

objections. (See guidelines on p. 206.)

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Arguments always contain premises - some premises will have support from other premises,—there are others that are mere assumptions (claims made by the arguer).

If the argument is valid/ strong, its soundness/cogency will turn on whether these assumptions are true.

So how can we tell if we should accept them?

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It is reasonable to accept a claim if… (1) The claim does not conflict with personal

experience that we have no good reason to doubt it.

(2) The claim does not conflict with background beliefs that we have no good reason to doubt.

(3) The claim comes from a credible source.

8

In general, you should favor the testimony of your own sight (and other senses) over the testimony of others.

Someone tells you her Doberman - gentle as a kitten.

You’ve seen him attack many people. You should probably not believe that the Doberman

is gentle.

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However, your senses are not indubitable (un-doubtable).

They can be mistaken for any number of reasons such as: Bad physical conditions (e.g., poor lighting) Sensory impairment (e.g., poor vision) Observer impairment (e.g., drunk) Unreliable measuring instruments Bad memory

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Just how areare you supposed to

to count how many times “are” appears in this quote?

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Background beliefs: convictions held—usually assumed without question. e.g., It snowed in Las Vegas last July 4th.

This seems to contradict our background belief that it doesn’t snow in deserts during the summer.

e.g., George W. Bush is a robot. This contradicts our background belief that

people aren’t robots.

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Nonetheless background beliefs should be subject to revision if sufficient evidence is presented against them (don’t be dogmatic about any beliefs you have).

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Much of what we believe is based on the testimony of sources.

We saw a lot about this in chapter 6 (more is in chapter 12).

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Questions to ask to determine source credibility:

Genuine expert? Are they outside their area? Are they biased? Do they have a reason to lie? Questionable senses (were they drunk)? Are they generally reliable (is it The Enquirer? )

Right context? Can expert opinion settle the issue (e.g., is this a

moral issue)? Is it improbable?

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There are two ways to refute an argument: Show that a premise—or a critical group of

premises—is false or dubious. Show that the conclusion does not follow

from the premises.

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If a premise is critical to an argument, showing it false will refute the argument.

(1) All presidents live in the White House. (2) Paris Hilton is President. So, (3) Paris Hilton lives in the White House.

Showing (2) to be false is sufficient to refute the argument.

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However, showing false an irrelevant premise will not refute the argument.

(1) All circles are squares. (2) All squares are rectangles. (3) All rectangles are geometrical figures. (4) So, all squares are geometrical figures.

Showing (1) is false won’t keep (2) and (3) from proving (4).

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Additionally, for refutation, the premise must be necessary (critical)

(1) TJ is a bachelor. (2) TJ is an uncle. (3) So, TJ is a Male.

Since both (1) and (2) provide independent support for (3) falsifying only one of them will not refute the argument.

21

If a critical premise is shown to be doubtable, then the argument has been refuted (by showing it to be unconvincing).

Demonstrating doubt: Appeal to personal experience, common knowledge,

or reputable source. Note a self-contradiction (either in a single premise

or between premises). Show the premises is based on an unwarranted

assumption. Personally demonstrate its falsity or dubiousness.

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Other refutation Techniques: Reducing to the absurd: Show the truth of a

premise would entail something clearly false (absurd).

Present a counter-example: present an exception that shows a premise false. (arguer): All 20th Century presidents were

rich. (you): Harry Truman wasn’t!

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To do this, you need to show that the argument is either (a) deductively invalid or (b) inductively weak.

Most important questions: If deductive, does the conclusion follow

necessarily from the premises? Are the premises relevant (is there a fallacy)? Are the premises sufficient to support the

conclusion? Does the argument omit any crucial

countervailing evidence? 24

Exercise 8.3 – Parts I, II and III

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