Psychology 3130A/B Chapter Notes - Chapter 7: Modus Ponens, Relate, Unconformity
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20 Dec 2017
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Chapter 7: Deductive Reasoning
Deduction and induction
• Induction involves making predictive inferences from observations, induction moves
from specific to general (based on evidence) and the conclusions are probabilistic
o Going beyond given evidence to discover something new via thinking
• Deduction starts with a general statement and then proceeds to more specific
statements
o Involves verifying that which is already known
• Preise: MDoald’s offee is hot
o Premise like this can be used to make precise conclusions
o For example, combined with additional premises and a conclusion, we can create
an entire categorical syllogism
• Preise: All MDoald’s offee is hot
• Preise: This offee is fro MDoald’s
• Conclusion: Therefore this coffee is hot
• In a deductive statement, its assumed that the premises are true
o Deduction is considered to be valid if the conclusion follows directly from the
premises
• A valid deduction is one where the conclusion is the only possible conclusion given the
premises
o Can be no other possible conclusions from these premises; if these true premises
allow for alternative conclusions, then the deduction is not valid
Structure of a logical task
• Previous McDonalds example shows how deduction can be used to arrive at a
conclusion about a member of a category
• Can rely on it to make predictions about options and outcomes
• Premise: Your friend is waiting at Starbucks or by the shoe store
• Premise: Your friend is not at Starbucks
• Conclusion: Therefore your friend is by the shoe store
• Deductive statement (syllogism) has several components
• Premise gives basic factual information that we can reason from and reason about
• Crucial aspect = assuming premises are true
• Operators are crucial to deduction task, part of what make this different from inductive
reasoning (previous example was word OR)
o Define the nature of the deduction and can modify the complexity of the
argument
Deduction can seem counterintuitive
• We may agree with a stated conclusion even if it is not logically valid
• Or may reject conclusions that are valid
• May agree with a valid conclusion, but for idiosyncratic reasons
• Failure to accept logical task: failing to reason logical
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o Fail to distinguish between a conclusion that was logically valid and one that was
factually correct or one that they agreed with
• “eeed to plae hea preiu o the otet of the sllogis ad did’t reaso
logically
• Premise: It is important to talk about the things that are on our mind
• Premise 2: Household problems are on our mind
• Conclusion: It is important to talk about household problems
• Hele’s poit, people ofte isuderstad hat it eas to e logial, plae a hea
premium on content
• Henle endorsed people seem not to accept logical tasks as being deducto-logical
o Misstate premises, omit premises, and generally fall prey to kinds of cognitive
biases
o Did ot treat as logial task, ee he direted to, does’t ea the a’t
think logically
• Fundamental paradox of rationality = Johnson-Laird suggests that naïve reasoners with
no training in formal thought may make many cognitive errors in reasoning and yet still
manage to achieve their goals and make good decisions
o Paradox because rationality should be a necessary condition for correct decision-
making ad should be hallmark of formal, mature thinking, and yet in many ways
it does not seem to be necessary at all
• May be that for many basic decisions and conclusions, logical deduction is not needed
and additional resources needed to reason correctly may be suboptimal
Categorical Reasoning
• Occurs when we make conclusions on the basis of category membership
o = classical reasoning, because we are reasoning about a class of things
• stronger similarity results on stronger inductions
• there is an emphasis on category membership rather than similarity
• e.g. of classical syllogism
o Major Premise: All men are mortal
o Minor Premise: Socrates is a man
o Conclusion: Therefore Socrates is mortal
• In the classical syllogism, the standard format is a major premise, which refers to a
statement about the category
• In the minor, statement offers specific information
• Little role for similarity or featural overlap in these statements
• Many varieties of formal classical syllogisms, we focus on 4
Universal affirmative
• Statement in which the relationship between two categories is universal for all members
• all ats are aials; All A are B
• it is not reflexive, and thus has 2 possible forms
• (1) all members of category A are contained within a larger category B
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