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Types of Arguments - PowerPoint PPT Presentation

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Types of Arguments
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  1. Types of Arguments Inductive Argument: An argument in which the truth of the premises is supposed to prove that the conclusion is probably true. Strong Inductive Argument: An inductive argument in which the truth of the premises really does prove that the conclusion is probably true.

  2. Weak Inductive Argument: An inductive argument in which the truth of the premises really does not prove that the conclusion is probably true. Cogent Argument: A strong inductive argument in which all the premises are actuallytrue. Uncogent Argument: A strong inductive argument in which even one premise is actually false.

  3. Determining Whether an Inductive Argument is Strong or Weak • Assume all the premises are true, even if one or more is clearly false. • Using only the information in the premises plus common knowledge ask: What is the probability that the conclusion is true? Above 50%: Strong Argument 50% or Less: Weak Argument

  4. Examples Most recording artists have talent. Britney Spears is a recording artist. Therefore, Britney probably has talent. This argument is strong. Britney Spears is a recording artist. Britney has blonde hair. Therefore, most recording artists have blonde hair. This argument is weak.

  5. Most boys like to play sports. Britney Spears is a boy. Therefore, Britney probably likes to play sports. This argument is strong but uncogent. The first argument is cogent as well as strong.

  6. Deductive Argument: An argument in which the truth of the premises is supposed to necessitate the truth of the the conclusion, i. e. if all the premises are true, the conclusion has to be true. Valid Deductive Argument: A deductive argument in which the truth of the premises really does necessitate the truth of the conclusion, i. e. it’s NOT possible for all the premises to be true and the conclusionstill be false.

  7. Invalid Deductive Argument: A deductive argument in which the truth of the premises really does not necessitate the truth of the conclusion, i. e. it’s possible for all the premises to be true and the conclusion still be false. Sound Argument: A valid deductive argument in which all the premises are actuallytrue. Unsound Argument: A valid deductive argument in which even one premise is actually false.

  8. Determining Whether a Deductive Argument is Valid or Invalid • Assume all the premises are true, even if one or more is clearly false. • Using only the information in the premises plus common knowledge ask: Does the conclusion have to be true? Yes: Valid Argument No:Invalid Argument

  9. N.B.: An argument is invalid if it’s possible for all the premises to be true while the conclusion is false. N.B.: Whether an argument is valid or invalid depends solely on its formNOT its content.

  10. Examples If Barack Obama is President of the USA, then Michelle Obama is First Lady. Barack is President of the USA. Therefore, Michelle is First Lady. This argument is valid. Material Implication: The truth of the antecedent (A) is sufficient by itself for the truth of the consequent (B).

  11. If A, then B. A. Therefore, B. A’s being true makes B true. A is true. So, B is also true. If Abraham Lincoln died of cancer, then Lincoln is dead today. Lincoln is dead today. Therefore, Abraham Lincoln died of cancer. (This argument is invalid.)

  12. If A, then B. B. Therefore, A. Counter-Example: A way to show that an argument form is invalid. It’s an argument in the which the all the premises are clearly true and the conclusion is clearly false.

  13. If the moon is made of green cheese, then astronauts can eat moon rocks. The moon is made of green cheese. Therefore, astronauts can eat moon rocks. If A, then B. A. Therefore, B. This argument is valid but unsound.