Loading presentation...

Present Remotely

Send the link below via email or IM


Present to your audience

Start remote presentation

  • Invited audience members will follow you as you navigate and present
  • People invited to a presentation do not need a Prezi account
  • This link expires 10 minutes after you close the presentation
  • A maximum of 30 users can follow your presentation
  • Learn more about this feature in our knowledge base article

Do you really want to delete this prezi?

Neither you, nor the coeditors you shared it with will be able to recover it again.


Informal Fallacies of Logic


Michael Pace

on 17 October 2016

Comments (0)

Please log in to add your comment.

Report abuse

Transcript of Informal Fallacies of Logic

Informal Fallacies of Logic
Fallacies of Presumption
Fallacies of Weak Induction
Fallacies of Relevance
Fallacies of Ambiguity & Category Errors
* Complex Question
* False Dilemma
* From Ignorance
* Slippery Slope
two choices are given when in fact there are three options
two unrelated points are conjoined as a single proposition
a series of increasingly unacceptable consequences is drawn
A limited number of options (usually two) is given, while in reality there are more options. A false dilemma is an illegitimate use of the "or" operator.
Putting issues or opinions into "black or white" terms is a common instance of this fallacy.

1. http://www.colbertnation.com/the-colbert-report-videos/403349/november-30-2011/barney-frank-s-better-know-a-district-replay (Bush-Great president or greatest president?)
2. Either you're for me or against me.
2. America: love it or leave it.
3. Either support Meech Lake or Quebec will separate.
4. Every person is either wholly good or wholly evil.
5. http://www.indecisionforever.com/2009/05/22/jon-stewart-contrasts-obama-and-cheneys-terror-speeches/ @3:00

Identify the options given and show (with an example) that there is an additional option.
In order to show that a proposition P is unacceptable, a sequence of increasingly unacceptable events is shown to follow from P. A slippery slope is an illegitimate use of the "if-then" operator.

If we pass laws against fully-automatic weapons, then it won't be long before we pass laws on all weapons, and then we will begin to restrict other rights, and finally we will end up living in a communist state. Thus, we should not ban fully-automatic weapons.
You should never gamble. Once you start gambling you find it hard to stop. Soon you are spending all your money on gambling, and eventually you will turn to crime to support your earnings.

Proof: Identify the proposition P being refuted and identify the final event in the series of events. Then show that this final event need not occur as a consequence of P.
Two otherwise unrelated points are conjoined and treated as a single proposition. The reader is expected to accept or reject both together, when in reality one is acceptable while the other is not. A complex question is an illegitimate use of the "and" operator.

You should support home education and the God-given right of parents to raise their children according to their own beliefs.
Do you support freedom and the right to bear arms?
Have you stopped using illegal sales practises? (This asks two questions: did you use illegal practises, and did you stop?)

Identify the two propositions illegitimately conjoined and show that believing one does not mean that you have to believe the other.
* Appeal to Force
* Red Herring
* Strawman
the author attacks an argument different from (and weaker than) the opposition's best argument
The arguer diverts attention by changing the subject to a different (but sometimes subtly related) one.
* Appeal to (Improper) Authority
1. the authority is not an expert in the field
2. experts in the field disagree
3. the authority was joking, drunk, or in some other way not being serious
Crimes of theft and robbery have been increasing at an alarming rate lately. The conclusion is obvious: we must reinstate the death penalty immediately.
There is a good deal of talk these days about the need to eliminate pesticides from our fruits and vegetables. But many of these foods are essential to our health. Carrots are an excellent sources of vitamin A, broccoli is rich in iron, and oranges and grapefruits have a lot of vitamin C.
Environmentalists are continually harping about the dangers of nuclear power. Unfortunately, electricity is dangerous no matter where it comes from. Every year hundreds of people are electrocuted by accident. Since most of these accidents are caused by carelessness, they could be avoided if people would just exercise greater caution.

Show that the premises are irrelevant to the truth of the conclusion (and that the fallacy isn't better described by one of the more specific fallacies involving irrelevant premises, e.g., ad hominem.)
The author attacks an argument which is different from, and usually weaker than, the opposition's best argument.

People who opposed the Charlottetown Accord probably just wanted Quebec to separate. But we want Quebec to stay in Canada.
We should have conscription. People don't want to enter the military because they find it an inconvenience. But they should realize that there are more important things than convenience.
Freedom of speech is not nearly as desirable as some people think. Why should anyone be allowed, for example, to incite others to violence, to make threatening phone calls, or to shout obscenities at policemen? And surely no one thinks people should be allowed to make fraudulent proposals, or to reveal military secrets to our enemies.
Marsha Taylor, the college president, argues that it's inappropriate to have condom-dispensing machines on campus. Apparently Taylor is completely anti sex. Just abolish sex altogether. That's what she wants. But without sex there would be no human race. And no animals either. Obviously Taylor's argument makes no sense whatsoever.

Show that the opposition's argument has been misrepresented by showing that the opposition has a stronger argument. Describe the stronger argument.
1. Noted psychologist Dr. Frasier Crane recommends that you buy the EZ-Rest Hot Tub.
2. Economist John Kenneth Galbraith argues that a tight money policy s the best cure for a recession. (Although Galbraith is an expert, not all economists agree on this point.)
4. My friend heard on the news the other day that Canada will declare war on Serbia. (This is a case of hearsay; in fact, the reporter said that Canada would not declare war.)
5. The Ottawa Citizen reported that sales were up 5.9 percent this year. (This is hearsay; we are not n a position to check the Citizen's sources.)

Show that either (i) the person cited is not an authority in the field, or that (ii) there is general disagreement among the experts in the field on this point.
International terrorist Osama Bin Laden planned the destruction of the World Trade Center, killing thousands of innocent people, and he supports terrorist causes all over the world. Bin Laden is therefore a wicked and irresponsible person.
Shakespeare cannot possibly have written the thirty-six plays attributed to him, because the real Shakespeare was a two-bit country businessman who barely finished the fourth grade in school and who never left the confines of his native England.
Mickey has testified that he saw Freddy set fire to the building. But Mickey was recently convicted on ten counts of perjury, and he hates Freddy with a passion and would love to see him in jail. Therefore, you should not believe Mickey’s testimony.
* Composition
* Equivocation
* Division
the same term is used with two different meanings
because the attributes of the parts of a whole have a certain property, it is argued that the whole has that property
because the attributes of the whole has a certain property, it is argued that the parts have that property
The same word is used with two different meanings.

1. Criminal actions are illegal, and all murder trials are criminal actions, thus all murder trials are illegal. (Here the term "criminal actions" is used with two different meanings. Example borrowed from Copi.)
2. The sign said "fine for parking here", and since it was fine, I parked there.
3. All child-murderers are inhuman, thus, no child-murderer is human. (From Barker, p. 164; this is called "illicit obversion")
4. A plane is a carpenter's tool, and the Boeing 737 is a plane, hence the Boeing 737 is a carpenter's tool. (Example borrowed from Davis, p. 58)

Identify the word which is used twice, then show that a definition which is appropriate for one use of the word would not be appropriate for the second use.
Because the parts of a whole have a certain property, it is argued that the whole has that property. That whole may be either an object composed of different parts, or it may be a collection or set of individual members.

1. The brick wall is six feet tall. Thus, the bricks in the wall are six feet tall.
2. Germany is a militant country. Thus, each German is militant.
3. Conventional bombs did more damage in W.W. II than nuclear bombs. Thus, a conventional bomb is more dangerous than a nuclear bomb. (From Copi, p. 118)

Show that the properties in question are the properties of the whole, and not of each part or member or the whole. If necessary, describe the parts to show that they could not have the properties of the whole.
Because the whole has a certain property, it is argued that the parts have that property. The whole in question may be either a whole object or a collection or set of individual members.

1. Each brick is three inches high, thus, the brick wall is three inches high.
2. Because the brain is capable of consciousness, each neural cell in the brain must be capable of consciousness.

Show that the properties in question are the properties of the parts, and not of the whole. If necessary, describe the parts to show that they could not have the properties of the whole.
Humourous Equivocations from Groucho Marx
“Time flies like an arrow, but fruit flies like a banana.”
“Outside of a dog, a book is man’s best friend. Inside of a dog, it’s too dark to read.”
“One morning I shot an elephant in my pajamas. How he got in my pajamas I’ll never know.”
* Hasty Generalization
* False Analogy
the sample is too small to support an inductive generalization or is unrepresentative of the whole
the two objects or events being compared are relevantly dissimilar
The size of the sample is too small to support the conclusion.

1. Fred, the Australian, stole my wallet. Thus, all Australians are thieves. (Of course, we shouldn't judge all Australians on the basis of one example.)
2. I asked six of my friends what they thought of the new spending restraints and they agreed it is a good idea. The new restraints are therefore generally popular.
3. One hundred thousand voters from Orange County, CA, were surveyed on their choice for governor, and 68 percent said they intended to vote for the Republican candidate. Clearly the Republican candidate will be elected.

Identify the size of the sample and the size of the population, then show that the sample size is too small. Note: a formal proof would require a mathematical calculation. This is the subject of probability theory. For now, you must rely on common sense.

Alternative: Show that there is good reason to doubt the second premise in this argument:
1. Some members of group G are F
2. If these members are F, then probably the whole group is F
3. Therefore, the whole group is F.
In an analogy, two objects (or events), A and B are shown to be similar. Then it is argued that since A has property P, so also B must have property P. An analogy fails when the two objects, A and B, are different in a way which affects whether they both have property P.

1. Employees are like nails. Just as nails must be hit in the head in order to make them work, so must employees.
2. Government is like business, so just as business must be sensitive primarily to the bottom line, so also must government.
3. http://www.indecisionforever.com/2009/12/17/foxs-laura-ingraham-godwins-the-tax-debate/

Identify the two objects or events being compared and the property which both are said to possess. Show that the two objects are different in a way which will affect whether they both have that property.
6. http://www.southparkstudios.com/clips/151029/
Note: Definitions of many fallacies are from http://onegoodmove.org/fallacy/toc.htm
the truth of the conclusion is assumed by the premises
* Begging the Question
The truth of the conclusion is assumed by the premises. Often, the conclusion is simply restated in the premises in a slightly different form. In more difficult cases, the premise is a consequence of the conclusion.

1. Since I'm not lying, it follows that I'm telling the truth.
2. We know that God exists, since the Bible says God exists. What the Bible says must be true, since God wrote it and God never lies. (Here, we must agree that God exists in order to believe that God wrote the Bible.)

Show that in order to believe that the premises are true we must already agree that the conclusion is true.
* Appeal to the People (Ad Populum)
* Appeal to Pity
The reader is told that unpleasant consequences will follow if they do not agree with the author.

1. You can't agree that evolution is true, because if it were, then we would be no better than monkeys and apes.
2. You must believe in God, for otherwise life would have no meaning. (Perhaps, but it is equally possible that since life has no meaning that God does not exist.)

Identify the consequences to and argue that what we want to be the case does not affect what is in fact the case.
A type of Appeal to Consequences in which the reader is persuaded to agree by force
the reader is persuaded to agree by sympathy

The reader is told to agree to the proposition because of the pitiful state of the author.


1. How can you say that's out? It was so close, and besides, I'm down ten games to two.
2. We hope you'll accept our recommendations. We spent the last three months working extra time on it.


Identify the proposition and the appeal to pity and argue that the pitiful state of the arguer has nothing to do with the truth of the proposition.
the reader is warned of unacceptable consequences
an attempt to persuade others by appealing to a desire be accepted or valued
because something is not known to be true, it is assumed to be false
Arguments of this form assume that since something has not been proven false, it is therefore true. Conversely, such an argument may assume that since something has not been proven true, it is therefore false. (This can be thought of as a special case of a false dilemma, since it assumes that all propositions must either be known to be true or known to be false.)

Since you cannot prove that ghosts do not exist, they must exist.
Since scientists cannot prove that global warming will occur, it probably won't.

Proof: Identify the proposition in question. Argue that it may be true even though we don't know whether it is or isn't.
None of Mr. Albie’s close friends have ever seen him walk without crutches. We conclude that Mr. Albie most likely needs crutches to walk.
NOTE: Not all arguments with that appeal to ignorance are fallacies
Controversial: No one knows for sure whether God exists. Therefore, God does not exist.
Appeal to Force
1. You had better agree that the new company policy is the best bet if you expect to keep your job.
2. NAFTA is wrong, and if you don't vote against NAFTA then we will vote you out of office.
* Ad Hominem (Attacking the Person)
Instead of directly addressing an argument by someone else, the fallacious reasoner points out features of the person giving the argument. Comes in three more specific versions:
1. Abusive: the person's character is attacked
2. Circumstantial: the person's circumstances are noted
3. Tu Quoque: the person does not practice what is preached
You may argue that God doesn't exist, but you are just following a fad. (ad hominem abusive)
William Buckley has argued in favor of legalizing drugs such as cocaine and heroin. But Buckley is just another one of those upper-crust intellectuals who is out of touch with real America. No sensible person should listen to his pseudo-solutions. (Abusive)
The Dalai Lama argues that China has no business in Tibet and that the West should do something about it. But the Dalai Lama just wants the Chinese to leave so he can return as leader. Naturally he argues this way. Therefore, we should reject his arguments. (Circumstantial)
Child to parent: Your argument that I should stop stealing candy from the corner store is no good. You told me yourself just a week ago that you, too, stole candy when you were a kid. (Tu Quoque)

Identify the attack and show that the character or circumstances of the person has nothing to do with the truth or falsity of the proposition being defended.
Note: Examples that are not ad hominems:
A proposition is held to be true because it is widely held to be true or is held to be true by some (usually upper crust) sector of the population. This fallacy is sometimes also called the "Appeal to Emotion" because emotional appeals often sway the population as a whole.

1. If you were beautiful, you could live like this, so buy Buty-EZ and become beautiful. (Here, the appeal is to the "beautiful people".)
2. I look out at you all, and I tell you, I am proud to be here. Proud to belong to a party that stands for what is good for America. Proud to cast my lot with the kind of people who make this nation great. Yes, there are those who criticize us, who label our view of trade agreements as "protectionist". But when I look at you hard-working people, I know that we're right and the critics are wrong.
@6:00 (Can you also spot an ad hominem circumstantial?)
"Were you surprised to learn that the president is gay?"
Full transcript