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Tutorials
| Prepared by: Minh Viet Le |
| Caufield School of Information Technology, |
| Faculty of Information Technology, |
| Monash University |
| Date created: May 2006 |
Premise(s) must be true in order for the conclusion to be true.
A good argument is the one that provides a good reason for accepting a claim.
If p then q.
p.
Therefore, q.
If p then q.
Not q.
Therefore, not p.
If p, then q.
q.
Therefore, p.
For example, if the alien spaceship landed, there should be a large circular depression in the field. There is a large circular depression in the field. So the alien spaceship must have landed.
If p, then q.
Not p,
Therefore, not q.
For example, if Michael wins a lottery, he will have a new car. He did not win a lottery. Therefore, he must not have a new car.
Phenomena p.
If hypothesis h were true, it would provide the best explanation for p.
Therefore, it's probable that h is true.
Arguing in a circle occurs when its conclusion is used as one of its premises. For example: "John is very rich." says Michael. "How do you know?" asks Mary. "Because he has a lot of money," replies Michael.
The fallacy of the biased sample is committed whenever the data for a statistical inference is drawn from a sample that is not representative of the population under consideration. For example: a recent survey by a university of its students, 80% respondents indicated their strong approval of the increase of number of opening hours in libraries. This survey shows clearly that increased number of opening hours in libraries will make everyone at its university happy.
The fallacy of the insufficient sample is committed whenever an inadequate sample is used to justify the conclusion drawn. For example, "My friends are migrated from country X and their spoken English is very bad. It is obvious that people from country X are very bad in spoken English. An inference from a sample of a group to the 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 part of the sample. Another example, "I have worked with three people from New York City and found them to be obnoxious, pushy and rude. It is obvious that people from New York City have a bad attitude.
Ad hominem means "to the man", indicates an attack that is made upon a person rather than upon the statement that person has made. For example: "Don't listen to my opponent; he's a homosexual".
For example, "The earth has air, water, and living organisms, Mars has air and water. Therefore, Mars has living organisms." The success of such arguments depends on the nature and extent of the similarities between the two objects. Another example: "John and Michael are friends and were born on the same day in the same year at the same time. John is very good at playing piano. Therefore, Michael must be very good at playing piano." The problem is that any two things have some features in common. Consequently an argument from analogy can be successful only if the dissimilarities between the things being compared are insignificant.
Here the speaker attributes an argument to an opponent that does not represent the opponent's true position. For instance, a political candidate might charge that his opponent "wants to let all prisoners go free," when in fact his opponent simply favors a highly limited furlough system. The person is portrayed as someone that he is not.
This is a "false cause" fallacy in which something is associated with something else because of mere proximity of time. People often claim, for example, that because something occurred after something else it is caused by it. For example, since you wore crystals around your neck you haven't caught a cold. Therefore, the crystals must help me stay healthy. In fact, there are a number of other factors that could be involved.
This is the so-called black or white fallacy. Essentially, it says "Either you believe what I'm saying, or you must believe exactly the opposite." The argument above assumes that there are only two possible alternatives open to us. For example, "Either science can explain how she was cured or it was a miracle. Science can't explain how she was cured. So it must be a miracle." These two alternatives do not exhaust all the possibilities. It's possible, for example, that she was cured by some natural cause that scientists don't yet understand. There is no room for a middle ground. Another example, "Objects were moving in the house. Either someone was moving them by psychokinesis or it was ghosts. It wasn't psychokinesis. Therefore, it must have been ghosts.
This fallacy is committed when it is assumed, without justification, that background conditions have remained the same at different times/locations. For example, "Every day that you've lived has been followed by another day that you've been alive. Therefore, every day you ever will live will be followed by another day that you will be alive. Another example, "You won this table tennis game last year. Therefore, you must win this year.
The fallacy of equivocation occurs when a word or phrase that has more than one meaning is employed in different meanings throughout the argument. For example, to say "(i) Only man is rational. (ii) No woman is a man. (iii) Therefore, no woman is rational."
Non Sequitor means "does not follow," which short for the conclusion does not follow from the premise. For example, to say "My mobile phone can be used to watch TV, therefore it must be able to play mp3 files.
The truth or falsity of an idea is determined not by where it came from, but by the evidence supporting it. For example: "Juan's idea is the result of a mystical experience, so it must be false (or true).
He is the professor (an expert but not in the relevant field) who has said that, so it must be true.
Everybody believes or does it, so it must be true. Just because a lot of people believe something or like something doesn't mean that it's true or good. A lot of people used to believe that the Earth was flat, but that certainly didn't make it so. Similarly, a lot of people used to believe that women should not have the right to vote. Popularity is not a reliable indication of either reality or value.
For example: "If you do not convict this criminal, one of you may be her next victim." This argument is fallacious because what a defendant might do in the future is irrelevant to determining whether she is responsible for a crime committed in the past. Threats extort; they do not help us arrive at the truth.
The appeal to ignorance comes in two varieties: using an opponent's inability to disprove a conclusion as proof of the conclusion's correctness, and using an opponent's inability to prove a conclusion as proof of its incorrectness.
In the first case, the claim is that since there is no proof that something is true, it must be false. For example: "There is no proof that the parapsychology experiments were fraudulent, so I'm sure they weren't."
In the second case, the claim is that since there is no proof that something is false, it must be true. For example: "Bigfoot must be exist because no one has been able to prove that he doesn't."
The problem with these arguments is that they take a lack of evidence for one thing to be good evidence for another. A lack of evidence, however, proves nothing.
We appeal to tradition when we argue that something must be true (or good) because it is part of an established tradition. For example: "Mothers have always used chicken soup to fight colds, so it must be good for you. The fact that people have always done or believed something is no reason for thinking that we should continue to do or believe something.
The division fallacy occurs when one assumes that what is true of a whole is also true of its parts. For example: "Society's interest in the occult is growing. Therefore Michael's interest in the occult is growing." Since groups can have properties that are not had by their members, such an argument is fallacious.