Bassham3 powerpoint lecturenotes_ch06

Lecture Notes © 2008 McGraw Hill Higher Education1

Critical Thinking

Ch 6Logical Fallacies IIFallacies of Insufficient Evidence


The Fallacies of Insufficient Evidence

Inappropriate Appeal to Authority Appeal to Ignorance False Alternatives Loaded Question Questionable Cause Hasty Generalizations Slippery Slope Weak Analogy Inconsistency


Inappropriate Appeal to Authority

…occurs when an arguer cites an authority who, there is good reason to believe, is unreliable.

Ways we can question reliability: Are they an authority/expert? Are they biased?

• Usually “someone having something to gain” is reason to doubt their claim; however it can’t affect the soundness of their argument.

Are their observations questionable? (on drugs?) Are they generally reliable? (Enquirer) Are they citing their source correctly? (mis-quote?) Does the authority disagree with expert consensus? Is it an issue that can be settled by an authority? It is highly improbable?


Appeal to Ignorance …occurs when someone claims that, the failure

to prove something false, entails that it is true (or visa-versa). e.g., There must not be intelligent life on other

planets. We have never found any. Exceptions:

Fruitless Searches: If a search is exhaustive (we looked everywhere), or extensive (we tested for years), then a lack of evidence can be sufficient evidence.

Special rules: e.g., innocent until proven guilty.


False Alternatives

Insisting that there are less choices than there actually are. e.g., You can either vote Republican or Democrat, but you

don’t want a Democrat, so you must vote Republican. Usually such arguments are in the form of “either/or” (like

the one above). But they can also present multiple options: e.g., when an argument says there are only three options

when there are actually four. They also can be in the form of an “if then.”

e.g., If we don’t elect a Democrat the economy will go down the tubes and we don’t want that! So we should elect a Democrat.


Loaded Question A loaded question is a question that contains a presupposition such that,

either way you answer it, you will appear to endorse that assumption. Examples:

Have you stopped cheating on your exams? Where did you hide the bodies? Are you still in favor of this fiscally irresponsible bill?

Usually they are multiple questions “rolled up” into one. The last question really means:

• Do you think the bill is fiscally irresponsible? • Did you support the bill?• Will you support the bill?

When someone asks you a loaded question the appropriate response is—not to answer it but—have them clarify what question they want answered (or clarify for them, and answer each individually).


Questionable Cause …occurs when one claims, without sufficient evidence, that

one thing is the cause of something else. The post hoc fallacy: suggesting that A causes B just

because A came before B. • e.g., I drank the ginseng tea and I was better by the next day. The

tea must have made me better. Mere correlation fallacy: suggesting that the constant

conjunction of A and B entails that they are causally related.• e.g., I every morning this week I ate eggs, and every day I failed an

exam. I should stop eating eggs so I can pass my exams. Oversimplified cause fallacy: suggesting that A is the cause

of B when clearly B has many causes. • e.g., SAT scores have been dropping. Kids have been watching too

much TV.


Hasty Generalization …occurs when one draws a general conclusion from a sample that is

biased or too small. Biased sample: I polled 100 professors from 100 schools, only 25% of

them believed in God. I guess most Americans don’t believe in God anymore.

Too small of a sample: I asked my professors if they believed in God, and only one did. I guess professors don’t believe in God anymore.

If it doesn’t have a “general conclusion,” then it’s not a generalization . That biker with the swastika tattoo and brass knuckles will probably beat

me up if I talk to him. Since this argument draws a conclusion about one biker, and not all (or

most) of them, it is not a “generalization” at all.


The Slippery Slope …fallacy is committed when one claims, without sufficient

evidence, that a seemingly harmless action will lead to a terrible one. e.g., Dr. Perry has proposed that we legalize physician-

assisted suicide. No sensible person should listen to such an proposal. If we allow physician-assisted suicide eventually there will be no respect for human life.

Common form: A leads to B, and B leads to C, and C to do D, and we really don’t want D. Thus, we shouldn’t do A.

Exception: If one presents good evidence that “A” will lead to “D,” and if

D should be avoided, then the conclusion that A should be avoided is justified.


Weak Analogy …occurs when an arguer compares two (or more) things that aren’t

really comparable in the relevant respect. e.g., Lettuce is leafy and green and good on burgers. Poison Ivy leafy

and green. It would be good on burgers too. Common forms:

A has characteristics w, x, y and z. B has characteristics w, x and y. Therefore, B probably has characteristic z too.

A is x and y. B is x and y. C is x. So C is y. Many exceptions:

Alice lives in a mansion and she is rich. Bruce lives in a mansion. Bruce is probably rich too.

The form is easy to spot, but—quite often—to know whether it is fallacy or not, you just have to know whether the shared characteristics are relevant to the concluded one.


Inconsistency …the fallacy of inconsistency is committed when an arguer

espouses two logically contradictory claims. e.g., Nobody goes there anymore; it’s too crowded.

Common form A and not A.

The only exceptions to this rule are equivocations: Bob is dead even though he isn’t.

• If you mean “he’s emotionally dead, even though he isn’t physically dead” then you are not contradicting yourself… you are just being unclear (by being ambiguous).

But the exact same thing can never be both true and false at the same time.