Reason

Reason

What is reason?

Deductive Reasoning

SyllogismsQuantifiers:-all -some-noSyllogisms must contain -two premises -a conclusion-three terms which occur twice

All dogs have legs, Charlie is a dog, Therefore Charlie has legs

All islands are surrounded by water,San Juan is an islandTherefore San Juan is surrounded by water

Some A’s are B’sSome B’s are C’sTherefore some A’s are C’s

Belief BiasPremises that are believed because the conclusion is correct even though the premises may be incorrect.

All pianos eat peanut butter,My dog is a piano,Therefore my dog must eat peanut butter

Venn Diagrams

All A’s are B’s Some A’s are C’sTherefore some B’s are C’s

B

C AI

All A’s are B’sAll B’s are C’sTherefore all C’s are A’s

C B A

Deductive reasoning preserves the truthAre the premises true?

Is the argument valid?

EnthymemeAn argument that has not been set out in a formal way.If a premise strikes the person using this argument as obvious they will simply assume that the premise is correct and not bother stating it.

Inductive Reasoning

Specific observations > general rules-“Every human I’ve observed is mortal; I’ve never observed a human who is immortal; therefore, all humans are mortal”

Opposite of deductive reasoning-“All humans are mortal; I’m a human; therefore, I’m mortal.”

What makes a good generalization?Number – number of instances/observationsVariety – how well your sample group represents the whole groupException – you should actively look for counterexamples Coherence – you should demand more evidence for things that are surprising / don’t fit with your current mental mapSubject area – generalizations are more reliable in natural sciences than social ones.

Informal Reasoning

What is informal reasoning?Informal Reasoning is reasoning that contains a fallacy.

Fallacies1. Ad ignorantiam2. Post hoc ergo propter hoc3. Ad hominem4. Circular reasoning5. Special pleading6. Equivocation7. False analogy8. False dilemma9. Loaded question

Argument ad ignorantiamIf you claim that something is true on the grounds that there is no evidence to disprove it, or visa versa.

Eg. The weather tomorrow will be rainy because you cannot prove it’s not true.

Post hoc ergo propter hocAssuming that because one thing follows another thing, the first thing is the cause of the second thingIf B follows A, then A is the cause of B.

Eg. I ate a piece of pizza earlier tonight. I have stomach now. Therefore, my stomach ache is caused by eating the pizza.

Ad hominem Attacking or supporting a person rather than their argument

Eg. It must be true because Einstein said so.

Circular reasoning Begging a questionAssuming truth in something you are supposed to be proving

Eg. I know that Jesus is the son of God because he said he is. And the son of God would not lie.

Special Pleading Double standardsMaking moral exceptions for yourself

Eg. “I know there is a drought and I need to save water, but I am putting my prize flowers in a competition next week and I need to give them plenty of water.”

Equivocation When a word is given two different meanings in an argument

Eg. A hamburger is better than nothing, Nothing is better than good health, Therefore a hamburger is better than good health.

False analogy To assume that because two things are similar in some respects, they must also be similar in further respects

Eg. Our eyes, ears, nose and mouth create 7 windows on our heads, therefore there must be 7 planets- Francesco Sizzi, 17th century astronomer

False Dilemma This is the fallacy of assuming that only two alternatives exist when there is in fact a wider-range of options. Seeing things in black and white.

Eg. You didn’t come to school today, so you must be sick at home.

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

Eg. Jeff! How many of my shirts did you steal?

Causes of bad reasoning1. Ignorance2. Laziness3. Pride4. Prejudice

Ignorance

Laziness

Pride

Prejudice