Evaluating Philosophical Claims
and Theories
Argument?
CR’s main campus is located at
the southern end of Eureka.
Obama is not trustworthy.
It is irrational to believe that
human spirits or souls exist
because there is no evidence
that justifies this belief.
Abortion should be made illegal in
all cases except when a woman’s life
is endangered by continuing the
pregnancy. Additionally, doctors
who perform abortions should be
charged with murder.
If Americans want to continue to be
competitive in the global economy,
then they need to decrease military
spending and increase funding for
public education.
Argument
“An argument is a set of
statements in which one or
more of the statements
attempts to provide reasons
or evidence of the truth of
another statement” (40).
• Premise.
• Conclusion.
• Inference.
It is irrational to believe
that human spirits or souls
exist because there is no
evidence that justifies this
belief.
Analyzing an Argument
1. Identify premise(s) and
conclusion.
2. Arrange the argument so
that the conclusion
appears after the
premise(s).
It is irrational to believe that
human spirits or souls exist
because there is no evidence
that justifies this belief.
There is no evidence that
justifies the belief that human
spirits and souls exist.
Therefore, it is irrational to
believe that human spirits or
souls exist.
3. Determine whether the
argument depends on any
unstated premises and, if
so, add to these to the
argument.
4. Determine whether the
argument is deductive or
inductive.
5. Deductive: determine
whether the argument is
valid.
6. Deductive: determine
whether the argument is
sound.
It is irrational to believe that
something exists if there is no
evidence that justifies the
belief.
There is no evidence that
justifies the belief that human
spirits and souls exist.
Therefore, it is irrational to
believe that human spirits or
souls exist.
irrationalto
tobelieve
believethat
that
ItItisisirrational
somethingexists
existsififthere
thereisis
something
noevidence
evidencethat
thatjustifies
justifies
no
the
thebelief.
belief.
Thereisisno
noevidence
evidencethat
that
There
justifiesthe
thebelief
beliefthat
that
justifies
human
humanspirits
spiritsand
andsouls
souls
exist.
exist.
Therefore,ititisisirrational
irrationalto
to
Therefore,
believe
believethat
thathuman
humanspirits
spirits or
or
souls
exist.
souls
exist.
• Conceptual Clarity.
It is irrational to believe
that something exists if
there is no evidence that
justifies the belief.
• Logical Inconsistency.
There is no evidence that
justifies the belief that
human spirits and souls
exist.
• Coherence.
Therefore, it is irrational to
believe that human spirits
or souls exist.
• Compatibility.
• Self-Referential
Inconsistency.
• Comprehensiveness.
“Philosophy is not simply the sharing of personal
opinions…The goal of philosophical reflection is
not simply to have emotionally satisfying beliefs,
but the goal is to have beliefs that are true” (45).
Evaluate: “The truth or
falsity of a belief is
determined by the person
holding the belief because
there is no objective truth,
only subjective truth.
Therefore, what is true to
one person may be false to
another person.”
“Evaluating philosophies
is not like tasting foods”
(37).
If the universe exhibits
qualities of design, the
universe had a designer.
If the universe exhibits
qualities of design, the
universe had a designer.
The universe exhibits
qualities of design.
The universe had a designer.
Therefore, the universe had a
designer.
Modus Pollens
If P, then Q.
P.
Therefore, Q.
Therefore, the universe
exhibits qualities of design.
Affirming the Consequent
(Invalid)
If P, then Q.
Q.
Therefore, P.
If the government imposes
import restrictions, the price
of automobiles will rise.
Therefore, since the
government will not impose
import restrictions, it follows
that the price of automobiles
will not rise.
If the government imposes import
restrictions, the price of
automobiles will rise.
Identify conclusion and
premise(s) and arrange for
analysis.
Valid?
The government will not impose
import restrictions.
Therefore, it follows that the price
of automobiles will not rise.
Denying the Antecedent
(Invalid)
If P, then Q.
Not-P.
Therefore, not-Q.
Using Counterexamples to Demonstrate Invalidity
(but not validity)
If the government imposes
import restrictions (P), then the
price of automobiles will rise (Q).
If Abraham Lincoln committed
suicide (P), then Abraham
Lincoln is dead (Q).
The government will not impose
import restrictions (not-P).
Abraham Lincoln did not commit
suicide (not-P).
Therefore, it follows that the
price of automobiles will not rise
(not-Q).
Therefore, Abraham Lincoln is not
dead (not-Q).
Denying the Antecedent (Invalid)
If P, then Q.
Not-P.
Therefore, not-Q.
If human rights are
recognized, then
civilization flourishes. If
equality prevails, then
civilization flourishes.
Therefore, if human
rights are recognized,
then equality prevails.
Verify by
Counterexample
If the air conditioner
is running, then there
is a supply of
electricity to the
house.
Valid?
True premises?
If the heater is
running, then there is
a supply of electricity
to the house.
Counterfeit Hypothetical
Syllogism (invalid)
If P, then Q.
If R, then Q.
Therefore, if P, then R.
Therefore, if the air
conditioner is
running, then the
heater is running.
If human rights are recognized
(P), then equality prevails (Q).
If equality prevails (Q), then
civilization flourishes (R).
Therefore, if human rights are
recognized (P), then
civilization flourishes (R).
Hypothetical Syllogism (Valid)
If P, then Q.
If Q, then R.
Therefore, if P, then R.
Either people are born with their
fundamental personality
characteristics or people develop
their fundamental personality
characteristics through their
experiences of their environment. I
believe people develop their
fundamental personality
characteristics through their
experiences of their environment.
Therefore, people are not born with
their fundamental personality
characteristics.
Invalid
Either P or Q.
Q.
Therefore, not-P.
Either people are born with their
fundamental personality
characteristics or people develop
their fundamental personality
characteristics through their
experiences of their environment. I
believe people do not develop their
fundamental personality
characteristics through their
experiences of their environment.
Therefore, people are born with
their fundamental personality
characteristics.
Valid
Either P or Q.
Not-Q.
Therefore, P.
Remember the process….
1. Identify premise(s) and
conclusion.
6. Check for
2. Arrange the argument so that • Conceptual Clarity.
the conclusion appears after
the premise(s).
• Consistency (Logical
Inconsistency & Self-Referential
Inconsistency).
3. Determine whether the
argument depends on any
unstated premises and, if so, • Coherence.
add to these to the argument.
• Comprehensiveness.
4. Determine whether the
argument is valid.
• Compatibility.
5. Determine whether the
argument is sound.
If College of the Redwoods fails to improve the quality of
instruction, then it is likely that CR will lose students to
other California Community Colleges in the years ahead.
• Not an argument.
It looks to me like College of the Redwoods is closed today and
not holding classes. Jerry said last night that if it is snowing when
we wake up in the morning, then College of the Redwoods will be
closed and will cancel classes, and sure enough, when I woke up
this morning, it was snowing.
If it is snowing, CR is closed
and not holding classes.
Modus Ponens (affirming
the antecedent): Valid
It is snowing.
If P, then Q.
P.
Therefore, Q.
CR is closed and not holding
classes.
It looks to me like College of the Redwoods is open today and holding
classes. Jerry said last night that if it is snowing when we wake up in
the morning, then College of the Redwoods will be closed and will
cancel classes, but when I woke up this morning, it was not snowing.
If it is snowing, CR is closed
and not holding classes.
Denying the Antecedent:
Invalid
It is not snowing.
If P, then Q.
Not-P.
Therefore, not-Q.
CR is open and holding
classes.
It must be snowing outside today. Jerry said last night that if it is
snowing when we wake up in the morning, then College of the
Redwoods will be closed and will cancel classes, and when I
woke up this morning I got a text from CR saying that campus is
closed and classes are cancelled.
If it is snowing, CR is closed
and not holding classes.
Affirming the Consequent:
Invalid
CR is closed and not
holding classes.
If P, then Q.
Q.
Therefore, P.
Therefore, it is snowing.
It must not be snowing outside today. Jerry said last night that if
it is snowing when we wake up in the morning, then College of
the Redwoods will be closed and will cancel classes, and when I
woke up this morning I got a text from CR saying that campus is
open and classes are being held.
If it is snowing, CR is closed
and not holding classes.
Modus Tollens (denying
the consequent): Valid
CR is open and holding
classes.
If P, then Q.
Not-Q.
Therefore, not-P.
Therefore, it is not snowing.
Human beings experience themselves as more than just a material being of
flesh, blood, bone, and electro-chemical events. If human beings possess a
non-material self or soul, then human beings will experience themselves as
more than just a material being of flesh, blood, bone, and electro-chemical
events. Therefore, human beings possess a non-material self or soul.
Prove invalid by counterexample?
If human beings possess a nonmaterial self or soul (P), then
human beings will experience
• If human beings have the ability to
themselves as more than just a
inhabit two different spatial
material being of flesh, blood,
locations at one time, then human
bone, and electro-chemical events
beings will know what is
(Q).
happening in two places at the
same moment.
Human beings do experience
themselves as more than just a
• Human beings know what is
material being of flesh, blood,
happening in two places at the
bone, and electro-chemical events
same moment.
(Q).
Therefore, human beings possess
a non-material self or soul (P).
• Therefore, human beings have the
ability to inhabit two different
spatial locations at one time.
Since the good, according to Plato, is that which furthers a
person’s real interest, it follows that in any given case when
the good is known, men will seek it.
(from Philosophy and the Human Spirit by Avrum Stroll and Richard Popkin).
If the good is that which
furthers men’s real interest,
men will seek it when it is
known.
The good is that which furthers
men’s real interest.
Therefore, in any given case
when the good is known, men
will seek it.
• Valid.
• Conceptual Clarity.
• Consistency (Logical Inconsistency
& Self-Referential Inconsistency).
• Coherence.
• Comprehensiveness.
• Compatibility.
To every existing thing God wills some good. Hence, since to
love any thing is nothing else than to will good to that thing, it
is manifest that God loves every thing that exists.
(from Summa Theologica by Thomas Acquinas)
If God wills some good to every
existing thing, then God loves
every existing thing.
• Valid.
God wills some good to every
existing thing.
• Consistency (Logical
Therefore, God loves every
thing that exists.
• Conceptual Clarity.
Inconsistency & SelfReferential Inconsistency).
• Coherence.
• Comprehensiveness.
• Compatibility.
Arguments and Statements
The City of Eureka should clear
out the area behind the
Bayshore Mall and destroy all
the homeless encampments.
The Bayshore Mall is a fantastic
shopping venue because it offers
great prices and a diversity of
stores.
Go to the Bayshore Mall and buy
some food at Pretzel Wetzel.
Is there a Pretzel Wetzel at the
Bayshore Mall?
The homeless living behind the
Bayshore Mall are responsible for
a wide variety of property crimes
and cause significant
environmental damage. They
should be forcibly removed.
Even though the common
perception is that property
crimes have increased recently in
Humboldt County, in fact, they
have actually decreased.
Validity
Either she’s running late
or she ditched me to hang
out with someone else.
Either she’s running late
or she ditched me to hang
out with someone else.
She’s running late.
She’s not running late.
Therefore, she didn’t ditch
me to hang out with
someone else.
Therefore, she’s ditched
me to hang out with
someone else.
Valid?
If I am alive, then I am
breathing.
If I am disliked by my
friends, then I won’t be
invited to their party.
I am breathing.
Therefore, I am alive.
I wasn’t invited to their
party.
Therefore, I am disliked by
my friends.
Valid
If it is Saturday, then the
school will be closed.
If it is Saturday, then the
school will be closed.
The school is closed.
It is Saturday.
Therefore, it is Saturday.
Therefore, the school will
be closed.
Valid?
If today is February 14,
then today is Valentine’s
Day.
If Jack was born, Jack will
die.
Jack won’t die.
Today is not February 14.
Therefore, today is not
Valentine’s Day.
Therefore, Jack wasn’t
born.