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Inductive and deductive reasoning


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Introduction to Philosophy

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Inductive and deductive reasoning

  1. 1. Inductive and deductive Reasoning PHIL200
  2. 2. Inductive and Deductive Arguments <ul><li>Philosophy is centered in the analysis and construction of arguments, which is called logic. </li></ul><ul><li>An argument is the supporting of a thesis ( the conclusion with reason (premises) </li></ul><ul><li>An argument consists of at least two statements: a statement to be supported, the conclusion and a statements that support it the conclusion. </li></ul><ul><li>This process of reasoning from premises to conclusion is known as inference. </li></ul>
  3. 3. Deductive and Inductive Reasoning <ul><li>Arguments are two types-deductive and inductive. </li></ul><ul><li>A deductive argument gives logically conclusive support to its conclusion. </li></ul><ul><li>An inductive argument gives probable support to its conclusion. </li></ul>
  4. 4. Inductive and Deductive Reasoning <ul><ul><li>Arguments based on experience or observation are best expressed inductively, </li></ul></ul><ul><ul><li>while arguments based on laws, rules, or other widely accepted principles are best expressed deductively. </li></ul></ul>
  5. 5. Deduction <ul><li>Deduction is the reasoning process that draws a conclusion from the logical relationship of two assertions, usually one broad judgment or definition and one more specific assertion, often an inference. </li></ul>
  6. 6. Deductive Argument <ul><li>The two assertions that found the basis of a deductive argument are called premises. </li></ul><ul><li>The are usually two premises to consider: </li></ul><ul><ul><li>A major premise and a minor premise. </li></ul></ul><ul><ul><li>1-Socrates is a Man </li></ul></ul><ul><ul><li>2-All men are Mortal </li></ul></ul><ul><ul><li>Therefore Socrates is Mortal </li></ul></ul>
  7. 7. Deductive Arguments <ul><li>If Philosophy leads to wisdom, then it is worth studying. </li></ul><ul><li>Philosophy leads to wisdom </li></ul><ul><li>Therefore Philosophy is worth studtying. </li></ul><ul><li>If P then Q </li></ul><ul><li>P </li></ul><ul><li>Therefore Q </li></ul>
  8. 8. Deductive Arguments <ul><li>A Deductive Argument that succeeds in providing logically conclusive support to its conclusion is said to be valid, one that fails is said to be invalid. </li></ul>
  9. 9. Deductive Argument <ul><li>If the two premises are constructed logically then the conclusion will also follow logically </li></ul><ul><li>We say that the deductive argument is valid, this does not necessarily mean that the conclusion is true or false, </li></ul><ul><li>Validity comes from a logical conclusion based on logically constructed premises. </li></ul>
  10. 10. Constructing a Deductive Argument <ul><li>When constructing a deductive argument, your task is to defend the truth of your premises. </li></ul><ul><li>If your argument is valid, i.e., logically constructed, then the reader must agree with your argument. </li></ul><ul><li>If the reader disagrees, then the reader must prove that one of the premises is not true. </li></ul>
  11. 11. Deductive Reasoning <ul><li>A deductive argument serves as the basis of an entire essay supporting the argument conclusion by supporting the premises of the essay. </li></ul><ul><li>In the previous example of Lincoln the writer has to provide evidence that Lincoln actually performed with courage and a clear purpose. </li></ul>
  12. 12. Deductive Reasoning <ul><li>Note that the major premise of a deductive argument is either a broad judgment or a definition. </li></ul><ul><li>Thus, the major premise must be supported based on values and beliefs that the writer expects the reader to share. </li></ul><ul><li>The minor premise is usually an inference about a particular person or situation, supported by evidence as with inductive argument. </li></ul>
  13. 13. Examples of Deduction <ul><li>Homosexuality is an immoral act, because it unnatural and is a contradiction to God’s law of procreation. </li></ul><ul><li>Major Premise: All acts that are unnatural and contradict God Laws are immoral. </li></ul><ul><li>Homosexuality contradicts God’s law of procreation </li></ul><ul><li>Homosexuality is immoral. </li></ul>
  14. 14. Deduction Example <ul><li>Homosexuality is an immoral act, because it unnatural and is a contradiction to God’s law of procreation. Thus, homosexuality should be banned. </li></ul><ul><li>Major Premise: All acts that are unnatural and contradict God’s Law are immoral and should be banned. </li></ul><ul><li>Homosexuality contradicts God’s law of procreation </li></ul><ul><li>Conclusion: Homosexuality is immoral and should be banned. </li></ul>
  15. 15. Reductio Ad Absurdum <ul><li>An Indirect Method to prove or establish a thesis. </li></ul><ul><li>You assume the opposite of what you want to prove and then show that it produces a false conclusion. Therefore your thesis must be true. </li></ul><ul><li>See Page 33 </li></ul><ul><li>. </li></ul>
  16. 16. Inductive Arguments <ul><li>Inductive Arguments are not truth preserving. An inductive argument cannot prove if the premises are true then the conclusion will also be true. It is intended to prove only probable support to the conclusion. </li></ul><ul><li>An inductive argument that succeeds in providing such probable support is said to be strong. An inductive argument that fails to provide such support is said to be weak. A strong argument with true premises is said to be cogent. </li></ul><ul><li>Page 34 </li></ul>
  17. 17. Enumerative Induction <ul><li>A common inductive argument form reasons from premises about a few members of the group to conclusions about the group as a whole.( example page 35) </li></ul><ul><li>The group generalized is the target group . </li></ul><ul><li>The observed or the known members of the group are called the sample . </li></ul><ul><li>To reach a reliable conclusion about the target group the sample should be large enough and representative of the whole group. </li></ul><ul><li>Drawing conclusions about a target group based on a sample that is too small is a common error known as hasty generalization. </li></ul>
  18. 18. Inference to the Best Explanation ( abduction) <ul><li>A Type of inductive reasoning. </li></ul><ul><li>We reason from premises about a state of affairs to reach a conclusion about a state of affairs. ( page 36) </li></ul><ul><li>Inference to the Best explanation is especially important in science, where scientists advance their theories or hypothesis to explain a set of data, then evaluating these explanations to see what is best. </li></ul><ul><li>The theory of planetary movements ( heliocentric ( sun-centered theory) as an alternative to earth centered (Ptolemaic view) </li></ul>
  19. 19. Fallacies of Reasoning <ul><li>Ad Hominem </li></ul><ul><li>This argument attacks the person instead of his position </li></ul><ul><li>Example: </li></ul><ul><ul><li>What she says about Johannes Kepler’s astronomy of the 1600′s must be just so much garbage. Do you realize she’s only fourteen years old? </li></ul></ul><ul><li>This attack may undermine the arguer’s credibility as a scientific authority, but it does not undermine her reasoning. </li></ul><ul><li>That reasoning should stand or fall on the scientific evidence, not on the arguer’s age or anything else about her personally. </li></ul>
  20. 20. Argument from Authority <ul><li>This argument appeals to authoritative figure as means of persuation. </li></ul><ul><li>You should believe in the death penalty because Plato believed in it </li></ul>
  21. 21. Arguing in Circles <ul><li>Begging the Question </li></ul><ul><li>A form of circular reasoning in which a conclusion is derived from premises that presuppose the conclusion. </li></ul><ul><li>Normally, the point of good reasoning is to start out at one place and end up somewhere new, namely having reached the goal of increasing the degree of reasonable belief in the conclusion. </li></ul><ul><li>The point is to make progress, but in cases of begging the question there is no progress. </li></ul><ul><li>Example: </li></ul><ul><li>See Example page 39 </li></ul>
  22. 22. Appeal to Ignorance <ul><li>The fallacy of appeal to ignorance comes in two forms: </li></ul><ul><ul><li>(1) Not knowing that a certain statement is true is taken to be a proof that it is false. </li></ul></ul><ul><ul><li>(2) Not knowing that a statement is false is taken to be a proof that it is true. </li></ul></ul><ul><li>The fallacy uses an unjustified attempt to shift the burden of proof. The fallacy is also called “Argument from Ignorance.” </li></ul><ul><li>Example: </li></ul><ul><li>Nobody has ever proved to me there’s a God, so I know there is no God. </li></ul><ul><li>This kind of reasoning is generally fallacious. It would be proper reasoning only if the proof attempts were quite thorough, and it were the case that if God did exist, then there would be a discoverable proof of this. </li></ul>
  23. 23. False Dilemma <ul><li>Unfairly presenting too few choices and then implying that a choice must be made among this short menu of choices commits the false dilemma fallacy. </li></ul><ul><li>Example: </li></ul><ul><li>I want to go to Scotland from London. I overheard McTaggart say there are two roads to Scotland from London: the high road and the low road. I expect the high road would be too risky because it’s through the hills and that means dangerous curves. But it’s raining now, so both roads are probably slippery. I don’t like either choice, but I guess I should take the low road and be safer. </li></ul><ul><li>This would be fine reasoning is you were limited to only two roads, but you’ve falsely gotten yourself into a dilemma with such reasoning. There are many other ways to get to Scotland. </li></ul>
  24. 24. Slippery Slop Argument <ul><li>This fallacy consists of arguing without good reasons, that taking a particular step will inevitably lead to another, normally catastrophic steps. </li></ul><ul><li>Example: </li></ul><ul><li>Mom: Those look like bags under your eyes. Are you getting enough sleep? </li></ul><ul><li>Jeff: I had a test and stayed up late studying. </li></ul><ul><li>Mom: You didn’t take any drugs, did you? </li></ul><ul><li>Jeff: Just caffeine in my coffee, like I always do. </li></ul><ul><li>Mom: Jeff! You know what happens when people take drugs! Pretty soon the caffeine won’t be strong enough. Then you will take something stronger, maybe someone’s diet pill. Then, something even stronger. Eventually, you will be doing cocaine. Then you will be a crack addict! So, don’t drink that coffee. </li></ul>
  25. 25. Straw Man <ul><li>You commit the straw man fallacy whenever you attribute an easily refuted position to your opponent, one that the opponent wouldn’t endorse, and then proceed to attack the easily refuted position (the straw man) believing you have undermined the opponent’s actual position. Example (a debate before the city council): </li></ul><ul><li>Opponent: Because of the killing and suffering of Indians that followed Columbus’s discovery of America, the City of Berkeley should declare that Columbus Day will no longer be observed in our city. </li></ul><ul><li>Speaker: This is ridiculous, fellow members of the city council. It’s not true that everybody who ever came to America from another country somehow oppressed the Indians. I say we should continue to observe Columbus Day, and vote down this resolution that will make the City of Berkeley the laughing stock of the nation. </li></ul><ul><li>The speaker has twisted what his opponent said; the opponent never said, nor even indirectly suggested, that everybody who ever came to America from another country somehow oppressed the Indians. </li></ul>
  26. 26. Genetic Fallacy <ul><li>A critic commits the genetic fallacy if the critic attempts to discredit or support a claim or an argument because of its origin (genesis) when such an appeal to origins is irrelevant. </li></ul><ul><ul><li>Example: </li></ul></ul><ul><ul><li>Whatever your reasons are for buying that DVD they’ve got to be ridiculous. You said yourself that you got the idea for buying it from last night’s fortune cookie. Cookies can’t think! </li></ul></ul>
  27. 27. Fallacies of Composition <ul><li>The composition fallacy occurs when someone mistakenly assumes that a characteristic of some or all the individuals in a group is also a characteristic of the group itself, the group “composed” of those members. It is the converse of the division fallacy. </li></ul><ul><ul><li>Example: </li></ul></ul><ul><ul><li>Each human cell is very lightweight, so a human being composed of cells is also very lightweight. </li></ul></ul>
  28. 28. Inconsistency <ul><li>The fallacy occurs when we accept an inconsistent set of claims, that is, when we accept a claim that logically conflicts with other claims we hold. </li></ul><ul><li>Example: </li></ul><ul><li>I’m not racist. Some of my best friends are white. But I just don’t think that white women love their babies as much as our women do. </li></ul><ul><li>That last remark implies the speaker is a racist, although the speaker doesn’t notice the inconsistency. </li></ul>