• When Claims Conflict
Contradictions - If a claim conflicts with
other claims we have good reason to
accept, we have good grounds for doubting
If a claim conflicts with our background
information, we have good reason to doubt
Chapter 4: Reasons for Belief and Doubt
Fact and Opinions
• Two meanings of “fact”
– State of affairs
– Claims that is true
• “That’s a matter of opinion”
– There are many different opinions about this issue
– There is no objective fact of the matter as the
grounds for deciding are entirely subjective
• Belief and Evidence
We should proportion our belief to the
It’s not reasonable to believe a claim
when there is no good reason for doing
• Experts and Evidence
If a claim conflicts with expert opinion,
we have good reason to doubt it.
Who is an expert?
Access to more information on the subject
than we do
They are better at judging that
information than we are.
When to rely on experts
• When they are experts in the relevant field
• When the are speaking or writing on their
subject of expertise
• When there is not some reason to doubt their
• When experts agree
• When the experts disagree about a claim, we
have good reason to doubt it.
Appeal to authority
• When any of the previous conditions is
violated the fallacy of appeal to authority is
• The reliance on expert opinion is undermined.
Some hints that the authority is dubious
• The expert is guilty of simple factual or formal errors.
• The expert’s claims conflict with what we have good
reason to believe.
• The expert does not adequately support his or her
• The expert’s writing contains logical contradictions or
• The expert is strongly biased, emotional, or dismissive.
• The expert relies on information you know is out of date.
• The other experts in the same field disagree.
•It’s reasonable to accept the evidence
provided by personal experience only if
there’s no good reason to doubt it.
• Factors that can give us good reason to
doubt the reliability of personal experience:
– We construct our memories
– We sometimes see what we want to see – the
constructive nature of perception
• Experiments showing that if we are told that
something is going to happen we will
• Stereotypes and perceptions:
• We (humans) are bad at making judgments
• We frequently make misjudgments about
• Gambler’s fallacy
So what conclusion should we draw from this?
Don’t rely solely on our intuitions.
• Resisting contrary evidence
• Looking for confirming evidence
• Preferring available evidence
Resisting contrary evidence
• We may deny, ignore, or reinterpret evidence
that contradicts cherished beliefs.
Looking only for confirming evidence
• Confirmation bias
• Popper’s point about confirmation – what
makes a theory scientific
Preferring available evidence
• Availability error
– Not just what is readily available in the sense of
easy to find, but also what is psychologically
– Weighting what you have seen or experienced
more heavily than it should be weighted
– Hasty generalization
• How to evaluate the reliability of
Consider whether the report conflicts
with what you have good reason to
Look for reporter slanting.
Consider the source.
Check for missing information.
Look for false emphasis.
Check alternative news sources.
Advertising and persuasion
• We generally have good reason to doubt
advertising claims and to be wary of
advertising’s persuasive powers.
– The purpose of advertising is to sell or promote
– History of misleading messages
• Snob appeal
– Ads asking you to identify with particular groups
– Insinuating that you will be identified with such
people if you….
• Catch phrases get our attention
• Vagueness of comparisons
• What’s the baseline with which we are
• Emptiness of claims
• Comparing “apples and oranges”
• Words which appear to make a strong claim
but are really close to lying
• Taking back what was said with modifiers that
undermine the claim