Evaluating Inductive
Reasoning
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A valid deductive argument guarantees
the truth of the conclusion, if the
premises are assumed as true. Hence,
deductive conclusions are known with
certainty.
Inductive arguments reach probable
conclusions. A strong inductive
argument makes a conclusion likely.
Inductive Examples

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The suspect has
chosen to remain
silent. Therefore,
she probably has
something to hide.
Three of four
cabbies interviewed
had BO. So cabbies
are not hygienic.

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A high pressure
zone is in place. So
it won’t rain today.
Since high doses of
saccharin are
correlated with
cancerous tumors in
rats, it will probably
produce cancer in
humans as well.
Assessing Inductive Strength
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
Every deductive argument is either valid
or invalid.
Inductive strength is a matter of
degree. An inductive argument may be
strong, moderate, or weak. If the
premises provided confer a high degree
of probability on the truth of the
conclusion, the argument is strong.
Which is Stronger?

Since the risk of
dying from FenPhen is less than the
risk of dying from
birth control pills,
we may conclude
that Fen-Phen is
relatively safe.

Some Fen-Phen
users experience
chest pain and
shortness of breath.
Therefore, since
Esther uses FenPhen, she probably
has chest pains.
Inductive Generalizations
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Reaching general conclusions from
premises that cite particular instances.
Is the sample group large enough? Is
the sample group representative?
In a survey of 500 nuns, 98% said they fully support
the Church’s position on birth control. It is therefore
likely that most Catholic women fully support the
church’s position on birth control.
Inductive Analogy

Analogical reasoning
involves comparing
similar cases and
inferring that what is
known to be true in
one case is likely to
be true in the similar
case.
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Like playing golf,
evaluating
arguments is a skill
that improves with
practice.
Analogue: playing
golf.
Target: evaluating
arguments.
Evaluating Analogies
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How many cases are compared?
How similar are the cases compared?
Are relevant dissimilarities overlooked?
A similarity is relevant when there is a
causal connection between the property
two cases share and the property that
is inferred to be true of the target.
Burden of Proof
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Where does the
burden of proof lie?
The less plausible a
claim is, the greater
the burden of proof.
The affirmative side
in a debate has a
greater burden of
proof.

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The greater the
chance of error, the
higher the cost of
being wrong, the
greater the burden
of proof.
Establish the
standard of proof.
Has it been met?
Hypothetical Reasoning


Reasoning from
observed facts to
explanatory
hypotheses. Why
does the
observational data
appear as it does?
Why did the
Challenger explode?



Why did the
dinosaurs become
extinct?
Why does
Fenfluramine
suppress appetite?
Why does
pregnancy cause
mood swings?
Hypothetical Reasoning


Observations: (1) 4 dead people in a
house. (2) A car running in a closed
garage. (3) An open heating vent from
the garage to the house.
Hypothesis: The people died because
of carbon monoxide poisoning.
Plausible Hypothesis


One that is likely to
survive critical
scrutiny.
Explanatory power:
How well does the
hypothesis explain
the observations?
Are there rival
hypotheses?
I’d say it’s logic anxiety.
Or it may be English-Angst
Predictive Power


Does the hypothesis
enable us to make
testable predictions?
If H is true, we will
observe O. We did
observe O.
Therefore, H is
confirmed.


If H is true, we will
observe O. We did
not observe O.
Therefore H is
disconfirmed.
Disconfirming
evidence is stronger
than confirming
evidence.
Causal Reasoning

Since causation
cannot be observed,
how can genuine
causal relationships
be distinguished
from mere
correlations?
So broccoli
eaters have
less cancer
and more
gas.
Mill’s Methods
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Method of agreement: Look for a
common antecedent condition in all
cases where the effect occurs; Where
the effect is found, the cause will be
present as well.
Ten students with diarrhea. Each had Tuna
Surprise at the cafeteria. The tuna was likely
the culprit.
If wherever there’s x there is also y, x may be the cause of y.
Method of Difference

Look for a correlation
between the absence of
the effect and the
absence of an
antecedent condition.
Where the effect does
not occur, the cause
won’t either.

If all those who
dined at the
cafeteria without
getting diarrhea also
did not eat the Tuna
Surprise, it is
reasonable to infer
that the Tuna was
responsible.
If whenever x does not occur, y is also not found, x may be the cause of y.
Joint method of Agreement
and Difference
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Look for an antecedent condition that
occurs whenever the effect occurs and
does not occur whenever the effect is
absent.
If they always get the runs after eating Tuna
Surprise, and never get the runs when they
don’t eat it, the case against the tuna is
strong.
If whenever there is x, there is y; and whenever there is not x, there is not y;
x is probably the cause of y.
Method of Concomitant
Variation

Look to see whether
variations in the
suspected cause are
correlated with
variations in the
effect.


Have Mikey eat two
helpings of Tuna
Surprise; have Ruby
eat one helping; and
give Charlie just one
bite.
Then watch what
happens!
If x causes y, then more of x will produce more of y.
Causal Claims

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A gun in the house
makes you safer.
Some cases of
mental dysfunction
are caused by
demon possession.
Playing violent video
games causes
violent behavior.

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Gay marriage harms
the institution of the
family.
Social acceptance of
homosexuality will
increase the number
of homosexuals in
society.
Marijuana use harms
health.
Verifying Premises

True premises
describe events
whose occurrence
can (in principle) be
verified or falsified.
Partial verification
produces degrees of
probability.

Clinton is celibate is
false (since the state
of affairs it describes
as existing does not
now exist) while
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The Democrats will
regain control of the
Senate is unknown.
Relativism
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There is no truth, just opinions.
A diversity of opinions does not entail
that all opinions are equally correct.
The facts may be elusive, but our
beliefs and desires do not determine
the nature of reality . A racist’s belief in
racial superiority does not make him
(even relatively) superior.
Relativism wrongly
concludes...

...that because we
may be mistaken,
we cannot be
correct. Our
inability to
absolutely prove one
view correct does
not show that all
opinions are equally
correct
A reasonable
person would
not make
that
inference.
Statement Types

Self-verifying
statements
(necessary truths,
analytic statements).
Nuns are females.
Tautologies: Either
cloning is a threat or
it isn’t a threat.

Empirically verifiable
statements:
observation
statements: It is
windy tonight.
Cloning of mammals
is possible. Juries
sometimes make
mistakes.
Verifying observation
statements

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Check the physical
conditions: Possible
perceptual
impairments
(Insufficient lighting,
time, etc.)
Check observer
accuracy (color
blind? on drugs?...)
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Observer
background
knowledge (of
astronomy, basketball, or whatever)
Observer objectivity
Corroboration of
other observers.
Empirically verifiable
hypotheses (or theories)
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Humans are descended from apes.
The universe began with a big bang.
Determine plausibility by examining
observational evidence-- does the
hypothesis have greater explanatory
power than its rivals? Does it have
predictive power?
Value Judgments
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Lying is wrong
You shouldn’t cheat on your spouse.
You should help the poor.
Democracy is better than fascism.
Can statements of value be verified?
Jeffrey ate his victim (verifiable).
Jeffrey is evil (unverifiable)
Justifying Value Judgments

PACE Debates
“Some of my best friends
are clones”
A rational process of
weighing arguments
and evidence that
aims to determine
whether or not there
is good reason to
accept a value
judgment.
Criteria for evaluating value
judgments
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Appeal to consequences: What are the
likely results? How will people, the
community etc. be affected? These are
factual issues.
Appeal to principles: Show that the act
or policy violates an established moral
principle.
Moral principles

Inflicting pain on an
unwilling subject for
personal pleasure is
immoral. This is
what eating meat
amounts to (since
we can survive
without it) Hence,
eating meat is
immoral.

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It is worse to punish
innocent people
than to allow guilty
people to escape
punishment.
Is this a plausible
moral principle?
Problems with value relativism
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Implies that value judgments can’t be
verified, but some value claims seem to
be more well established than some
factual claims. E.g..:
(a) Child abuse is wrong.
(b) Child abuse is caused by the
abuser’s upbringing.
Reasoning about values

The aim is not
absolute certainty,
but establishing
plausibility. This is
not merely a
question of personal
or societal
In this
preference.
village
adultery is a virtue. Just tell her
you’re from the east Sahara.
Religious Premises

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Religious premises are
always controversial
because there is no
agreed upon method for
settling religious
disputes.
Quotations from an
ancient scripture may
express profound
wisdom. (Eg., the
golden rule.)

However, ancient texts
also reflect unexamined
assumptions that arise
in particular historical
circumstances. Hence,
a quote from the Bible
can never constitute
conclusive evidence for
resolving a
contemporary moral
controversy.