Understanding Deductive Reasoning & Logical Fallacies in Reasoning, Slides of Brain and Cognitive Science

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Deductive Reasoning

• Propositional reasoning: drawing conclusions from premises (propositions)
• Logical connectives
  • AND
  • OR
  • NOT
  • IF-THEN

Some correct inferences

• Modus ponens

If P, then Q P ____________ Therefore, Q

• Modus tollens

If P, then Q Not Q _________ Therefore, not P

Wason’s 4-card task

• Turn over whichever cards are necessary to test the rule: “If a card has a vowel on one side, then it has an even number on the other side.”

A D 4 7

A somewhat simpler task

• Each card has an age and a drink on it. Turn over as many cards as necessary to test the rule, “If a person is drinking beer, then that person is 21 years of age or older.”

Beer Coke 21 18

Hypothesis testing

• The string “2, 4, 6” follows the rule. Generate other strings to guess the rule.
• Confirmation bias (Wason)

Difficulties in reasoning

• Negation (“not”) adds difficulty
• Misinterpretation of premises (“All A are B” does not imply that all B are A)
• Failure to consider all possibilities
• Content and believability effects