Logic And Fallacies: Constructing A Logical Argument

While I was search the tubes for to find the difference between a Rolls-Royce automobile and a Bentley automobile (my Dad told me there were only two differences: the badges and the hood ornament), I came across the following about constructing logical arguments.

An argument consists of one or more premises and one conclusion. A premise is a statement (a sentence that is either true or false) that is offered in support of the claim being made, which is the conclusion (which is also a sentence that is either true or false).

There are two main types of arguments: deductive and inductive.

A deductive argument is an argument such that the premises provide (or appear to provide) complete support for the conclusion.

An inductive argument is an argument such that the premises provide (or appear to provide) some degree of support (but less than complete support) for the conclusion.

If the premises actually provide the required degree of support for the conclusion, then the argument is a good one.

A good deductive argument is known as a valid argument and is such that if all its premises are true, then its conclusion must be true.

If all the argument is valid and actually has all true premises, then it is known as a sound argument.

If it is invalid or has one or more false premises, it will be unsound.

A good inductive argument is known as a strong (or "cogent") inductive argument. It is such that if the premises are true, the conclusion is likely to be true.

A fallacy is, very generally, an error in reasoning. This differs from a factual error, which is simply being wrong about the facts. To be more specific, a fallacy is an "argument" in which the premises given for the conclusion do not provide the needed degree of support.

A deductive fallacy is a deductive argument that is invalid (it is such that it could have all true premises and still have a false conclusion).

An inductive fallacy is less formal than a deductive fallacy. They are simply "arguments" which appear to be inductive arguments, but the premises do not provided enough support for the conclusion. In such cases, even if the premises were true, the conclusion would not be more likely to be true.

Following are 42 common fallacies used in the argumentative process and examples of each. Clicking on the fallacy will bring you to a link that presents it in more detail.

Ad Hominem:
Bill: "I believe that abortion is morally wrong."
Dave: "Of course you would say that, you're a priest."
Bill: "What about the arguments I gave to support my position?"
Dave: "Those don't count. Like I said, you're a priest, so you have to say that abortion is wrong. Further, you are just a lackey to the Pope, so I can't believe what you say."

Ad Hominem Tu Quoque:
Peter: "Based on the arguments I have presented, it is evident that it is morally wrong to use animals for food or clothing."
Bill: "But you are wearing a leather jacket and you have a roast beef sandwich in your hand! How can you say that using animals for food and clothing is wrong!"

Appeal to Authority:
I'm not a doctor, but I play one on the hit series "Bimbos and Studmuffins in the OR." You can take it from me that when you need a fast acting, effective and safe pain killer there is nothing better than MorphiDope 2000. That is my considered medical opinion.

Appeal to Belief:
God must exist. After all, I just saw a poll that says 85% of all Americans believe in God.

Appeal to Common Practice:
Director Jones is in charge of running a state's waste management program. When it is found that the program is rife with corruption, Jones says "This program has its problems, but nothing goes on in this program that doesn't go on in all state programs."

Appeal to Consequences of a Belief:
"I acknowledge that I have no argument for the existence of God. However, I have a great desire for God to exist and for there to be an afterlife. Therefore I accept that God exists."

Appeal to Emotion:
Bill goes to hear a politician speak. The politician tells the crowd about the evils of the government and the need to throw out the peoople who are currently in office. After hearing the speach, Bill is full of hatred for the current politicians. Because of this, he feels good about getting rid of the old politicians and accepts that it is the right thing to do because of how he feels.

Appeal to Fear:
"You know, Professor Smith, I really need to get an A in this class. I'd like to stop by during your office hours later to discuss my grade. I'll be in your building anyways, visiting my father. He's your dean, by the way. I'll see you later."

Appeal to Flattery:
"That was a wonderful joke about AIDS boss, and I agree with you that the damn liberals are wrecking the country. Now about my raise..."

Appeal to Novelty:
The Sadisike 900 pump-up glow shoe. It's better because it's new.

Appeal to Pity:
Jill: "He'd be a terrible coach for the team."
Bill: "He had his heart set on the job, and it would break if he didn't get it."
Jill: "I guess he'll do an adequate job."

Appeal to Popularity:
"My fellow Americans...there has been some talk that the government is overstepping its bounds by allowing police to enter peoples' homes without the warrants traditionally required by the Constitution. However, these are dangerous times and dangerous times require appropriate actions. I have in my office thousands of letters from people who let me know, in no uncertain terms, that they heartily endorse the war against crime in these United States. Because of this overwhelming approval, it is evident that the police are doing the right thing."

Appeal to Ridicule:
"Support the ERA? Sure, when the women start paying for the drinks! Hah! Hah!"

Appeal to sprite:
Bill: "I think that Jane did a great job this year. I'm going to nominate her for the award."
Dave: "Have you forgotten last year? Remember that she didn't nominate you last year."
Bill: "You're right. I'm not going to nominate her."

Appeal to tradition:
Of course the city manager form of government is the best. We have had this government for over 50 years and no one of any substance has talked about changing it in all that time. So, it has got to be good.

Bandwagon:
Bill thinks that welfare is needed in some cases. His friends in the Young Republicans taunt him every time he makes his views known. He accepts their views in order to avoid rejection.

Begging the question:
"If such actions were not illegal, then they would not be prohibited by the law."

biased sample:
Bill is assigned by his editor to determine what most Americans think about a new law that will place a federal tax on all modems and computers purchased. The revenues from the tax will be used to enforce new online decency laws. Bill, being technically inclined, decides to use an email poll. In his poll, 95% of those surveyed opposed the tax. Bill was quite surprised when 65% of all Americans voted for the taxes.

Burden of Proof:
"You cannot prove that God does not exist, so He does."

Circumstantial Ad Hominem:
"She asserts that we need more military spending, but that is false, since she is only saying it because she is a Republican."

Composition:
Sodium and Chloride are both dangerous to humans. Therefore any combination of sodium and chloride will be dangerous to humans.

Confusing Cause and Effect:
It is claimed by some people that severe illness is caused by depression and anger. After all, people who are severely ill are very often depressed and angry. Thus, it follows that the cause of severe illness actually is the depression and anger. So, a good and cheerful attitude is key to staying healthy.

Division:
"Sodium chloride (table salt) may be safely eaten. Therefore its constituent elements, sodium and chloride, may be safely eaten."

False Dilemma:
Bill: "Jill and I both support having prayer in public schools."
Jill: "Hey, I never said that!"
Bill: "You're not an atheist are you Jill?"

Gambler's Fallacy":
Bill is playing against Doug in a WWII tank battle game. Doug has had a great "streak of luck" and has been killing Bill's tanks left and right with good die rolls. Bill, who has a few tanks left, decides to risk all in a desperate attack on Doug. He is a bit worried that Doug might wipe him out, but he thinks that since Doug's luck at rolling has been great Doug must be due for some bad dice rolls. Bill launches his attack and Doug butchers his forces.

Genetic Fallacy:
"Sure, the media claims that Senator Bedfellow was taking kickbacks. But we all know about the media's credibility, don't we."

Guilt By Association:
"Will and Kiteena are arguing over socialism. Kiteena is a pacifist and hates violence and violent people.
Kiteena: "I think that the United States should continue to adopt socialist programs. For example, I think that the government should take control of vital industries."
Will: "So, you are for state ownership of industry."
Kiteena: "Certainly. It is a great idea and will help make the world a less violent place."
Will: "Well, you know Stalin also endorsed state ownership on industry. At last count he wiped out millions of his own people. Pol Pot of Cambodia was also for state ownership of industry. He also killed millions of his own people. The leadership of China is for state owned industry. They killed their own people in that square. So, are you still for state ownership of industry?"
Kiteena: "Oh, no! I don't want to be associated with those butchers!"

Hasty Generalization:
"Sam is riding her bike in her home town in Maine, minding her own business. A station wagon comes up behind her and the driver starts beeping his horn and then tries to force her off the road. As he goes by, the driver yells "get on the sidewalk where you belong!" Sam sees that the car has Ohio plates and concludes that all Ohio drivers are jerks

Ignoring A Common Cause:
"Over the course of several weeks the needles from the pine trees along the Wombat river fell into the water. Shortly thereafter, many dead fish washed up on the river banks. When the EPA investigated, the owners of the Wombat River Chemical Company claimed that is it was obvious that the pine needles had killed the fish. Many local environmentalists claimed that the chemical plant's toxic wastes caused both the trees and the fish to die and that the pine needles had no real effect on the fish

Middle Ground:
"A month ago, a tree in Bill's yard was damaged in a storm. His neighbor, Joe, asked him to have the tree cut down so it would not fall on Joes new shed. Bill refused to do this. Two days ago another storm blew the tree onto Joe's new shed. Joe demanded that Joe pay the cost of repairs, which was $250. Bill said that he wasn't going to pay a cent. Obviously, the best solution is to reach a compromise between the two extremes, so Bill should pay Joe $125 dollars.

Misleading Vividness:
Jane: "Did you hear about that woman who was attacked in Tuttle Park?"
Sarah: "Yes. It was terrible."
Jane: "Don't you run there everyday?"
Sarah: "Yes."
Jane: "How can you do that? I'd never be able to run there!"
Sarah: "Well, as callous as this might sound, that attack was out of the ordinary. I've been running there for three years and this has been the only attack. Sure, I worry about being attacked, but I'm not going give up my running just because there is some slight chance I'll be attacked."
Jane: "That is stupid! I'd stay away from that park if I was you! That woman was really beat up badly so you know it is going to happen again. If you don't stay out of that park, it will probably happen to you!"

Personal Attack:
Bill: "I don't think it is a good idea to cut social programs."
Jill: "Why not?"
Bill: "Well, many people do not get a fair start in life and hence need some help. After all, some people have wealthy parents and have it fairly easy. Others are born into poverty and..."
Jill: "You just say that stuff because you have a soft heart and an equally soft head."

Poisoning the Well:
"I ask you to remember that those who oppose my plans do not have the best wishes of the city at heart."

Post Hoc:
"I had been doing pretty poorly this season. Then my girlfriend gave me this neon laces for my spikes and I won my next three races. Those laces must be good luck...if I keep on wearing them I can't help but win!"

Questionable Cause:
Joe gets a chain letter that threatens him with dire consequences if he breaks the chain. He laughs at it and throws it in the garbage. On his way to work he slips and breaks his leg. When he gets back from the hospital he sends out 200 copies of the chain letter, hoping to avoid further accidents.

Red Herring"We admit that this measure is popular. But we also urge you to note that there are so many bond issues on this ballot that the whole thing is getting ridiculous."

Relativist Fallacy:
"That may be true for you, but it is not true for me."

Slippery Slope:
"You can never give anyone a break. If you do, they'll walk all over you."

Special Pleading:
Bill and Jill are married. Both Bill and Jill have put in a full day at the office. Their dog, Rover, has knocked over all the plants in one room and has strewn the dirt all over the carpet. When they return, Bill tells Jill that it is her job to clean up after the dog. When she protests, he says that he has put in a full day at the office and is too tired to clean up after the dog.

Spotlight:
Ann: "I'm not letting little Jimmy use his online account anymore!"
Sasha: "Why not? Did he hack into the Pentagon and try to start world war three?"
Ann: "No. Haven't you been watching the news and reading the papers? There are perverts online just waiting to molest kids! You should take away your daughter's account. Why, there must be thousands of sickos out there!"
Sasha: "Really? I thought that there were only a very few cases."
Ann: "I'm not sure of the exact number, but if the media is covering it so much, then most people who are online must be indecent."

Straw Man:
"Senator Jones says that we should not fund the attack submarine program. I disagree entirely. I can't understand why he wants to leave us defenseless like that."

Two Wrongs Make A Right:
"Bill says that capital punishment is fine, since those the state kill don't have any qualms about killing others."

Oh, by the way, it is considered bad form to point out a fallacy to a fallacy user---I mean nothing makes a person madder than telling them their logic is flawed. Sort of like saying, "I don't want to make you mad, but...."

The source for the material in this post is www.nizkor.org.