Bassham3 powerpoint lecturenotes_ch06


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Bassham3 powerpoint lecturenotes_ch06

  1. 1. Critical Thinking Ch 6 Logical Fallacies II Fallacies of Insufficient Evidence
  2. 2. The Fallacies of Insufficient Evidence <ul><li>Inappropriate Appeal to Authority </li></ul><ul><li>Appeal to Ignorance </li></ul><ul><li>False Alternatives </li></ul><ul><li>Loaded Question </li></ul><ul><li>Questionable Cause </li></ul><ul><li>Hasty Generalizations </li></ul><ul><li>Slippery Slope </li></ul><ul><li>Weak Analogy </li></ul><ul><li>Inconsistency </li></ul>
  3. 3. Inappropriate Appeal to Authority <ul><li>… occurs when an arguer cites an authority who, there is good reason to believe, is unreliable. </li></ul><ul><li>Ways we can question reliability: </li></ul><ul><ul><li>Are they an authority/expert? </li></ul></ul><ul><ul><li>Are they biased? </li></ul></ul><ul><ul><ul><li>Usually “someone having something to gain” is reason to doubt their claim; however it can’t affect the soundness of their argument. </li></ul></ul></ul><ul><ul><li>Are their observations questionable? (on drugs?) </li></ul></ul><ul><ul><li>Are they generally reliable? (Enquirer) </li></ul></ul><ul><ul><li>Are they citing their source correctly? (mis-quote?) </li></ul></ul><ul><ul><li>Does the authority disagree with expert consensus? </li></ul></ul><ul><ul><li>Is it an issue that can be settled by an authority? </li></ul></ul><ul><ul><li>It is highly improbable? </li></ul></ul>
  4. 4. Appeal to Ignorance <ul><li>…occurs when someone claims that, the failure to prove something false, entails that it is true (or visa-versa). </li></ul><ul><ul><li>e.g., There must not be intelligent life on other planets. We have never found any. </li></ul></ul><ul><li>Exceptions: </li></ul><ul><ul><li>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. </li></ul></ul><ul><ul><li>Special rules: e.g., innocent until proven guilty. </li></ul></ul>
  5. 5. False Alternatives <ul><li>Insisting that there are less choices than there actually are. </li></ul><ul><ul><li>e.g., You can either vote Republican or Democrat, but you don’t want a Democrat, so you must vote Republican. </li></ul></ul><ul><li>Usually such arguments are in the form of “either/or” (like the one above). But they can also present multiple options: </li></ul><ul><ul><li>e.g., when an argument says there are only three options when there are actually four. </li></ul></ul><ul><li>They also can be in the form of an “if then.” </li></ul><ul><ul><li>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. </li></ul></ul>
  6. 6. Loaded Question <ul><li>A loaded question is a question that contains a presupposition such that, either way you answer it, you will appear to endorse that assumption. </li></ul><ul><li>Examples: </li></ul><ul><ul><li>Have you stopped cheating on your exams? </li></ul></ul><ul><ul><li>Where did you hide the bodies? </li></ul></ul><ul><ul><li>Are you still in favor of this fiscally irresponsible bill? </li></ul></ul><ul><li>Usually they are multiple questions “rolled up” into one. </li></ul><ul><ul><li>The last question really means: </li></ul></ul><ul><ul><ul><li>Do you think the bill is fiscally irresponsible? </li></ul></ul></ul><ul><ul><ul><li>Did you support the bill? </li></ul></ul></ul><ul><ul><ul><li>Will you support the bill? </li></ul></ul></ul><ul><li>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). </li></ul>
  7. 7. Questionable Cause <ul><li>… occurs when one claims, without sufficient evidence, that one thing is the cause of something else. </li></ul><ul><ul><li>The post hoc fallacy : suggesting that A causes B just because A came before B. </li></ul></ul><ul><ul><ul><li>e.g., I drank the ginseng tea and I was better by the next day. The tea must have made me better. </li></ul></ul></ul><ul><ul><li>Mere correlation fallacy : suggesting that the constant conjunction of A and B entails that they are causally related. </li></ul></ul><ul><ul><ul><li>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. </li></ul></ul></ul><ul><ul><li>Oversimplified cause fallacy : suggesting that A is the cause of B when clearly B has many causes. </li></ul></ul><ul><ul><ul><li>e.g., SAT scores have been dropping. Kids have been watching too much TV. </li></ul></ul></ul>
  8. 8. Hasty Generalization <ul><li>… occurs when one draws a general conclusion from a sample that is biased or too small. </li></ul><ul><ul><li>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. </li></ul></ul><ul><ul><li>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. </li></ul></ul><ul><li>If it doesn’t have a “general conclusion,” then it’s not a generalization . </li></ul><ul><ul><li>That biker with the swastika tattoo and brass knuckles will probably beat me up if I talk to him. </li></ul></ul><ul><ul><li>Since this argument draws a conclusion about one biker, and not all (or most) of them, it is not a “generalization” at all. </li></ul></ul>
  9. 9. The Slippery Slope <ul><li>… fallacy is committed when one claims, without sufficient evidence, that a seemingly harmless action will lead to a terrible one. </li></ul><ul><ul><li>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. </li></ul></ul><ul><li>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. </li></ul><ul><li>Exception: </li></ul><ul><ul><li>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. </li></ul></ul>
  10. 10. Weak Analogy <ul><li>… occurs when an arguer compares two (or more) things that aren’t really comparable in the relevant respect. </li></ul><ul><ul><li>e.g., Lettuce is leafy and green and good on burgers. Poison Ivy leafy and green. It would be good on burgers too. </li></ul></ul><ul><li>Common forms: </li></ul><ul><ul><li>A has characteristics w, x, y and z. B has characteristics w, x and y. Therefore, B probably has characteristic z too. </li></ul></ul><ul><ul><li>A is x and y. B is x and y. C is x. So C is y. </li></ul></ul><ul><li>Many exceptions: </li></ul><ul><ul><li>Alice lives in a mansion and she is rich. Bruce lives in a mansion. Bruce is probably rich too. </li></ul></ul><ul><li>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. </li></ul>
  11. 11. Inconsistency <ul><li>… the fallacy of inconsistency is committed when an arguer espouses two logically contradictory claims. </li></ul><ul><ul><li>e.g., Nobody goes there anymore; it’s too crowded. </li></ul></ul><ul><li>Common form </li></ul><ul><ul><li>A and not A. </li></ul></ul><ul><li>The only exceptions to this rule are equivocations: </li></ul><ul><ul><li>Bob is dead even though he isn’t. </li></ul></ul><ul><ul><ul><li>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). </li></ul></ul></ul><ul><li>But the exact same thing can never be both true and false at the same time. </li></ul>