Starts by examining particulars and arriving
at a generalization that represents
Whenever we observe something around us
and then draw a conclusion about this, we
are engaging in inductive reasoning.
We should always ask three
questions of an inductive argument:
1. Is the evidence sufficient?
2. Is it representative?
3. Is it up-to-date?
When you jump from the observations
you’ve made or the examples you’ve
found and reason that the same is true of
This reasoning is not necessarily faulty, but
you have to be careful not to commit the
fallacy of Hasty Generalization.
Difference between sound inductive
• An inference from a sample of a group to
a whole group is legitimate only if the
sample is representative —that is, only if
the sample is sufficiently large and every
member of the group has an equal chance
to be a part of the sample.
A Deductive Argument:
• Is intended to provide conclusive support
for its conclusion.
• If it succeeds in providing conclusive
support it is valid.
• If it fails to support its conclusion, it is
Valid Deductive Arguments:
• If the premises are true, then the conclusion
MUST be true.
• It is impossible for a deductively valid argument
to have true premises and a false conclusion.
• Thus, we must distinguish whether or not the
premises are indeed true, since just because an
argument is in valid form does not necessarily
mean the conclusion is true; if the premises are
false, then the conclusion may also be false.
Example of a valid deductive
argument that is false:
Major premise: If you have scars on your
body, then you have been abducted by
Minor premise: You obviously do have scars
on your body.
Conclusion: Therefore, you have been
abducted by space aliens.
(This argument is valid, but it is not sound).
Example of a valid and sound
All minors are under the age of eighteen.
My son is under the age of eighteen.
Therefore, my son is a minor.
Consider the following
Penelope claims that all her test scores have been good, and so
her course grade should be good, too.
We can express this argument as a syllogism:
All those who receive good test scores receive good
Penelope’s received good test scores.
Therefore, Penelope gets a good course grade.
Is it valid? Is it sound?
When a premise is omitted…
• Some deductive arguments might seem
confusing or misleading because they are
missing a premise (usually the major
• We need to evaluate the truth of the
unexpressed premise in order to decide if
the logic is truly sound. (This is similar to
examining the implicit warrants in an
• Results when only two parts of the
• Thus, if a deductive argument is missing
a premise (if it is implied but not explicitly
stated) then the resulting syllogism is
called an enthymeme.
Example of an enthymeme:
Susan studies rhetoric, so I know she
What is left out here is the major premise
that all those who study rhetoric speak
The gun has the defendant's fingerprints on
the trigger. He is clearly guilty!
What is missing here, is the major premise
that links the gun to the crime, such as it
being found next to the murdered person's
body or that bullets from the body match
What could go wrong with this
"If you have been healed or saved or blessed
through TBN and have not contributed . . .
you are robbing God and will lose your
reward in heaven."
(Paul Crouch, co-founder of the Trinity Broadcasting Network, quoted
by William Lobdell, The Week, August 10, 2007)
What’s the missing premise?
It's morally wrong to treat human beings as
So, genetically engineering human beings is
"Hungry? Grab a Snickers."
(advertising slogan for Snickers chocolate bar)
"One of the Soviet Georgia's senior citizens
thought Dannon was an excellent yogurt.
She ought to know. She's been eating
yogurt for 137 years."
(1970s television advertisement for Dannon Yogurt)
"I wanted to serve as President because
I love this country and because I love
the people of this Nation."
(Jimmy Carter, 1980 Concession Address)
Final thoughts on enthymemes…
When a part of the argument is missing, it is assumed
not only to be true, but so obvious that it is not worth
This makes it very difficult to challenge, as questioning the
obvious is an admission of ignorance, which puts
oneself lower down the social order and opens oneself
It also uses and 'out of sight, out of mind' principle: when
the unsafe part of the argument is missed out, then
people may not realize that it has been omitted.
Advertisers and politicians thus make great use of
enthymemes (as we’ve seen in some of the previous
Let’s take apart some
arguments and find some
The Earth has air, water, and
Mars has air and water.
Therefore Mars has living
Ever since I started wearing these crystals
around my neck I haven’t caught a cold.
I bought some crystals for all my family
members so that they will also stay
The following appeared in a letter from the principal of Allentown High School
to the teachers in the school:
Backusville High School instituted a policy last year
requiring all of its students to arrive at school half an
hour early to finish all their homework before
attending classes. Since the inception of this policy,
15 percent more students have enrolled at
Backusville. The Board of Education reminds us that
the more students we have enrolled at Allentown
High School, the more federal funding we receive, so
it’s clear to improve the quality of education for all
Allentown students we need to institute an early-
attendance policy similar to the one that Backusville
Name that fallacy…
Q: Now, the United States government says that you are still funding military training
camps here in Afghanistan for militant, Islamic fighters and that you're a sponsor of
international terrorism... Are these accusations true?
Osama Bin Laden: ...At the time that they condemn any Muslim who
calls for his right, they receive the highest top official of the Irish
Republican Army at the White House as a political leader, while
woe, all woe is the Muslims if they cry out for their rights. Wherever
we look, we find the US as the leader of terrorism and crime in the
world. The US does not consider it a terrorist act to throw atomic
bombs at nations thousands of miles away, when it would not be
possible for those bombs to hit military troops only. These bombs
were rather thrown at entire nations, including women, children and
elderly people and up to this day the traces of those bombs remain
in Japan. The US does not consider it terrorism when hundreds of
thousands of our sons and brothers in Iraq died for lack of food or
medicine. So, there is no base for what the US says and this saying
does not affect us...
Source: "CNN March 1997 Interview with Osama bin Laden"
• Tu Quoque (two wrongs make a right) is a very
common fallacy in which one attempts to defend
oneself or another from criticism by turning the
critique back against the accuser.
• This is a classic Red Herring since whether the
accuser is guilty of the same, or a similar, wrong
is irrelevant to the truth of the original charge.
However, as a diversionary tactic, Tu Quoque
can be very effective, since the accuser is put on
the defensive, and frequently feels compelled to
defend against the accusation.
Some of you may have seen the 90-minute ABC network television show…
entitled "Growing Up in the Age of AIDS".… I was one of nine guests on
that live program.… …[A] single 45-second sound bite cost me a long
journey and two hectic days in New York City.
Why…did I travel to The Big Apple for such an insignificant role? …I felt a
responsibility to express the abstinence position on national television.…
How sad that adolescents hear only the dangerous "safe sex" message from
adults who should know better.
What follows, then, is what I would have said on television.…
Q: Why, apart from moral considerations, do you think teenagers should be taught to
abstain from sex until marriage?
…[N]ot one of 800 sexologists at a recent conference raised
a hand when asked if they would trust a thin rubber
sheath to protect them during intercourse with a known
HIV infected person. … And yet they're perfectly willing
to tell our kids that "safe sex" is within reach and that
they can sleep around with impunity.
Source: James C. Dobson, in a fund-raising letter for "Focus on the Family", February 13, 1992.