PSYC 256 Lecture Notes - Lecture 19: Syllogism, Smart People, Deductive Reasoning

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13 Jun 2018
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REASONING
Formal Reasoning
Deductive Reasoning
o Reasoning from a general rule to a specific case
E.g., Newton’s 3 Laws
Information → Pattern → Tentative hypothesis → Theory
o The process of applying the principles of formal logic to reach a decision or
evaluate the truth of a claim based on valid premises
E.g., rules of algebra
Syllogistic Reasoning
Syllogism: a three-statement logical form, with the first two parts stating the
premises and the third part stating a conclusion
o Particular form
One abstract form example:
o All A are B (premise)
o All B are C (premise)
o Therefore, all A are C (conclusion)
Premise: statement taken to be true
o Helps establish what is already known and has quantifier attached to it
(all/some/none)
Conclusion: if premises are valid, conclusion must be valid
To say syllogism is valid, is not to say it’s true
o Arguing against truth not validity
All professors are smart → all smart people watch Archer →
Therefore, all professors watch Archer
Rather, the conclusion logically follows premises
Confirmation bias
o Tendency to allow beliefs to affect evaluation of conclusions
o Prone to accept ones we agree with
o Prone to reject ones we disagree with
o Premises might be untrue
Propositional/Conditional Reasoning (logical)
Evaluate validity of an if-then proposition
o If this is an apple, then it is a fruit.
This is an apple.
Therefore this is a fruit
Conditional Reasoning: involves a logical determination of whether the evidence
supports, refutes, or is irrelevant to the state if-then relationship
Antecedent: the if clause in standard conditional reasoning (if-then) tasks
Consequent: the then statement in conditional reasoning
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Affirm (true)
Deny (false)
Antecedent
This is an apple. Therefore it is a
fruit (modus ponens)
Valid conclusion
This is not and apple, therefore this is
not a fruit.
Illogical - untrue
Consequent
This is a fruit. Therefore, it is an
apple.
Illogical - untrue
This is not a fruit, therefore is is not
an apple. (modus tollens)
Valid conclusion
Wasch Selection Task
Better in concrete situations than abstract situations
Affirm the antecedent
o Turn over E - if vowel - should have ach number on other side
Deny the consequent
o Turn over 8 - then add - should not have a vowel on other side
E, K, 8, 7
Reasoning Errors
Difficulties with abstract information
o Much harder when premises deal with abstract information
o Proper conclusions in concrete situations
Form Errors
o People sometimes draw incorrect conclusions simply by using one of the two
invalid forms; either denying the antecedent or affirming the consequent
Illicit Conversion
o People have a tendency to reverse the proposition in the if and then. They
then proceed to evaluate the given evidence against the now reversed
conditional
Search Errors
o People often don’t search for evidence but instead rely on a first impression
or on the first example
Confirmation bias: only searching for positive evidence
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