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Reason
This document defines and provides examples of different types of reasoning, including deductive, inductive, informal reasoning, and common fallacies. Deductive reasoning uses syllogisms with quantified statements and terms to make valid logical deductions. Inductive reasoning makes generalizations based on specific observations. Informal reasoning contains fallacies, the most common of which are presented: ad ignorantiam, post hoc ergo propter hoc, ad hominem, circular reasoning, special pleading, equivocation, false analogy, false dilemma, and loaded questions. Bad reasoning can be caused by ignorance, laziness, pride, and prejudice.
Reason
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- 4. Syllogisms Quantifiers: -all -some -no Syllogisms must contain -two premises -a conclusion -three terms which occur twice
- 5. All dogs havelegs, Charlie is a dog, Therefore Charlie has legs
- 6. All islands aresurrounded by water, San Juan is an island Therefore San Juan is surrounded by water
- 7. Some A’s areB’s Some B’s are C’s Therefore some A’s are C’s
- 8. Belief Bias Premisesthat are believed because the conclusion is correct even though the premises may be incorrect.
- 9. All pianos eatpeanut butter, My dog is a piano, Therefore my dog must eat peanut butter
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- 11. All A’s areB’s Some A’s are C’s Therefore some B’s are C’s
- 12. All A’s areB’s All B’s are C’s Therefore all C’s are A’s
- 13. Deductive reasoning preservesthe truth Are the premises true? Is the argument valid?
- 14. Enthymeme An argumentthat 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.
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- 16. 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.”
- 17. What makes agood generalization? Number – number of instances/observations Variety – how well your sample group represents the whole group Exception – you should actively look for counterexamples Coherence – you should demand more evidence for things that are surprising / don’t fit with your current mental map Subject area – generalizations are more reliable in natural sciences than social ones.
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- 19. What is informalreasoning? Informal Reasoning is reasoning that contains a fallacy. Fallacies 1. Ad ignorantiam 2. Post hoc ergo propter hoc 3. Ad hominem 4. Circular reasoning 5. Special pleading 6. Equivocation 7. False analogy 8. False dilemma 9. Loaded question
- 20. Argument ad ignorantiam If 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.
- 21. Post hoc ergopropter hoc Assuming that because one thing follows another thing, the first thing is the cause of the second thing If 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.
- 22. Ad hominem Attackingor supporting a person rather than their argument Eg. It must be true because Einstein said so.
- 23. Circular reasoning Begginga question Assuming 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.
- 24. Special Pleading Doublestandards Making 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.”
- 25. Equivocation When aword 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.
- 26. False analogy Toassume 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
- 27. False Dilemma Thisis 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.
- 28. Loaded question Aquestion that is biased because it contains a built-in assumption. Eg. Jeff! How many of my shirts did you steal?
- 29. Causes of badreasoning 1. Ignorance 2. Laziness 3. Pride 4. Prejudice
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Editor's Notes
- #11 Venn diagrams can be used to verify a syllogism. Drawing out the syllogism with a venn diagram will help picture it
- #13 Careful with interpreting them, this one is not true
- #14 If you begin with the truth you will end with the truth. Only true if the premise is true
- #15 Used during everyday life, incomplete argument