Informal reasoning. The more churches there are in a city, the more prostitutes there are

Informal reasoning

The more churches there are in a city, the more prostitutes there

are.

People who have a tattoo are more likely to develop lung cancer. This

is true.

Chris says that he knows someone who is 90 and who has smoked 40 cigarettes a day for 75 years and

he’s OK, so smoking can’t be harmful.

Kate bought a good luck charm and she seems to be doing

better at school.

Post hoc ergo propter hoc

Assuming one thing causes another just because one follows another.

“Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press

A summary of common fallacies

Ad ignorantiam

Claiming that something is true because it cannot be proved to be false.

“Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press

Hasty generalisation

Generalising from insufficient evidence.

“Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press

Ad hominem

Attacking /supporting the person rather than the argument.

“Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press

Circular reasoning

Assuming the truth of what you are supposed to be proving.

“Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press

Special pleading

Using double standards to excuse an individual or group.

“Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press

Equivocation

Using language ambiguously.

“Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press

False analogy

Assuming that because two things are alike in one way that they are alike in other respects.

“Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press

False dilemma

Assuming that only a limited number of alternatives exist.

“Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press

Loaded question

A question that is biased because it contains a built-in assumption.

“Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press

In each of the following cases, decide which best applies to the

argument.Hasty generalisation, post hoc ergo propter hoc, circular reasoning, ad hominem, special pleading, ad ignorantium, false dilemma, false analogy, equivocation, loaded question.

1.

Since strict gun control laws were introduced in Dodge City, the crime rate has risen. This shows that gun control does nothing to reduce crime.

“Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press

Hasty generalisation, post hoc ergo propter hoc, circular reasoning, ad hominem, special pleading, ad ignorantium, false dilemma, false analogy, equivocation, loaded question.

post hoc ergo propter hoc

Since strict gun control laws were introduced in Dodge City, the crime rate has risen. This shows that gun control does nothing to reduce crime.

2.

Mrs Bunker said she trusted me, and she must be telling the truth because she wouldn’t lie to someone that she trusted.

“Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press

Hasty generalisation, post hoc ergo propter hoc, circular reasoning, ad hominem, special pleading, ad ignorantium, false dilemma, false analogy, equivocation, loaded question.

circular reasoning

Mrs Bunker said she trusted me, and she must be telling the truth because she wouldn’t lie to someone that she trusted.

“Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press

3.

The ends justifies the means. After all, if you want to make omelettes, you have to break eggs.

“Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press

Hasty generalisation, post hoc ergo propter hoc, circular reasoning, ad hominem, special pleading, ad ignorantium, false dilemma, false analogy, equivocation, loaded question.

false analogy

The ends justifies the means. After all, if you want to make omelettes, you have to break eggs.

4.

That can’t be right, non of my friends would believe it.

“Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press

Hasty generalisation, post hoc ergo propter hoc, circular reasoning, ad hominem, special pleading, ad ignorantium, false dilemma, false analogy, equivocation, loaded question.

ad hominem

That can’t be right, non of my friends would believe it.

5.

Since many great scientists have believed in God, there must be some truth in religion.

“Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press

Hasty generalisation, post hoc ergo propter hoc, circular reasoning, ad hominem, special pleading, ad ignorantium, false dilemma, false analogy, equivocation, loaded question.

ad hominem

Since many great scientists have believed in God, there must be some truth in religion.

6.

Jordan and Roxanne got on very well on their two dates together. They are clearly well suited and should get married.

“Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press

Hasty generalisation, post hoc ergo propter hoc, circular reasoning, ad hominem, special pleading, ad ignorantium, false dilemma, false analogy, equivocation, loaded question.

hasty generalisation

Jordan and Roxanne got on very well on their two dates together. They are clearly well suited and should get married.

7.

Do you want to be part of the solution, or part of the problem?

Hasty generalisation, post hoc ergo propter hoc, circular reasoning, ad hominem, special pleading, ad ignorantium, false dilemma, false analogy, equivocation, loaded question.

“Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press

false dilemma

Do you want to be part of the solution, or part of the problem?

8.

“I agree that you shouldn’t copy mp3s illegally, but I’ve only copied a few and I do buy tracks sometimes.”

“Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press

Hasty generalisation, post hoc ergo propter hoc, circular reasoning, ad hominem, special pleading, ad ignorantium, false dilemma, false analogy, equivocation, loaded question.

special pleading

“I agree that you shouldn’t copy mp3s illegally, but I’ve only copied a few and I do buy tracks sometimes.”

9.

The average UK family has 2.5 children. The Dineens are average people, they must have 2.5 kids.

“Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press

Hasty generalisation, post hoc ergo propter hoc, circular reasoning, ad hominem, special pleading, ad ignorantium, false dilemma, false analogy, equivocation, loaded question.

equivocation

The average UK family has 2.5 children. The Dineens are average people, they must have 2.5 kids.

10.

Since no-one has been able to prove that God didn’t create the universe, we must conclude that God did make everything.

“Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press

Hasty generalisation, post hoc ergo propter hoc, circular reasoning, ad hominem, special pleading, ad ignorantium, false dilemma, false analogy, equivocation, loaded question.

ad ignorantium

Since no-one has been able to prove that God didn’t create the universe, we must conclude that God did make everything.

11.

Are all your family stupid, or is it just you?

“Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press

Hasty generalisation, post hoc ergo propter hoc, circular reasoning, ad hominem, special pleading, ad ignorantium, false dilemma, false analogy, equivocation, loaded question.

loaded question

Are all your family stupid, or is it just you?

12.

No scandal has ever touched Mr Rick Perry. He must be a clean living and honest man.

“Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press

Hasty generalisation, post hoc ergo propter hoc, circular reasoning, ad hominem, special pleading, ad ignorantium, false dilemma, false analogy, equivocation, loaded question.

ad ignorantium

No scandal has ever touched Mr Rick Perry. He must be a clean living and honest man.

13.

Just as you are more likely to take care of a car that you own rather than a car that you rent, a slave owner would look after a slave better than an employer would look after their worker.

“Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press

Hasty generalisation, post hoc ergo propter hoc, circular reasoning, ad hominem, special pleading, ad ignorantium, false dilemma, false analogy, equivocation, loaded question.

false analogy

Just as you are more likely to take care of a car that you own rather than a car that you rent, a slave owner would look after a slave better than an employer would look after their worker.

14.

To ignore the possibility that Joseph landed the first person on the moon just because nobody has heard of a Joseph’s space program is arrogant. If we are unaware of something does not mean that it never happened.

“Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press

Hasty generalisation, post hoc ergo propter hoc, circular reasoning, ad hominem, special pleading, ad ignorantium, false dilemma, false analogy, equivocation, loaded question.

ad ignorantium

To ignore the possibility that Norway landed the first person on the moon just because nobody has heard of a Norwegian space program is arrogant. If we are unaware of something does not mean that it never happened.

Causes of bad reasoning

• Ignorance: do not realize the reasoning is fallacious

• Laziness: too much effort to check the evidence. Psychologically easier to hold simple beliefs

• Pride: once we are in an argument we are more interested in winning then truth

• Prejudice: manufacture bad reasons to justify them

Conclusion

• Rationalist philosophers like Rene Descartes believed that reason was a way of knowing that can give us certainty.

• But it is only as certain as the premises on which it is based - and it is always possible that we have reasoned badly in arguing from premises to conclusions.

• We also raised various philosophical doubts about induction and deduction but in practice it is difficult to see how we can do without these two ways of reasoning.

Conclusion

• It seems that reason like other ways of knowing is a double edged tool.

• We need to reason to develop consistent beliefs about the world but we can become trapped in the ‘prison of logic’ and this can stifle our creativity.

• Furthermore, reason is not appropriate in every situation, and if someone is too rational they may simply come across as a cold and unfeeling robot.

• Reason needs to be balanced by emotion.