Psychology 3130A/B Chapter Notes - Chapter 7: Modus Ponens, Relate, Unconformity

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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
Preise: MDoald’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
Preise: All MDoald’s offee is hot
Preise: This offee is fro MDoald’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
“eeed to plae hea preiu o the otet of the sllogis ad 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
Hele’s poit, people ofte isuderstad hat it eas to e logial, plae 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 logial task, ee he direted 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 aials; 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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