eBook - ePub
Introduction to Logic
Harry J Gensler
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eBook - ePub
Introduction to Logic
Harry J Gensler
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Ă propos de ce livre
Introduction to Logic is clear and concise, uses interesting examples (many philosophical in nature), and has easy-to-use proof methods. Its key features, retained in this Third Edition, include:
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- simpler ways to test arguments, including an innovative proof method and the star test for syllogisms;
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- a wide scope of materials, suiting it for introductory or intermediate courses;
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- engaging examples, from philosophy and everyday life;
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- useful for self-study and preparation for standardized tests, like the LSAT;
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- a reasonable price (a third the cost of some competitors); and
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- exercises that correspond to the free LogiCola instructional program.
This Third Edition:
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- improves explanations, especially on areas that students find difficult;
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- has a fuller explanation of traditional Copi proofs and of truth trees; and
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- updates the companion LogiCola software, which now is touch friendly (for use on Windows tablets and touch monitors), installs more easily on Windows and Macintosh, and adds exercises on Copi proofs and on truth trees. You can still install LogiCola for free (from http://www.harryhiker.com/lc or http://www.routledge.com/cw/gensler).
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1
Introduction
1.1 Logic
Logic1 is the analysis and appraisal of arguments. Here weâll examine reasoning on philosophical areas (like God, free will, and morality) and on other areas (like backpacking, water pollution, and football). Logic is a useful tool to clarify and evaluate reasoning, whether on deeper questions or on everyday topics.
1 Key terms (like âlogicâ) are introduced in bold. Learn each key term and its definition.
Why study logic? First, logic builds our minds. Logic develops analytical skills essential in law, politics, journalism, education, medicine, business, science, math, computer science, and most other areas. The exercises in this book are designed to help us think more clearly (so people can better understand what weâre saying) and logically (so we can better support our conclusions).
Second, logic deepens our understanding of philosophy â which can be defined as reasoning about the ultimate questions of life. Philosophers ask questions like âWhy accept or reject free will?â or âCan one prove or disprove Godâs existence?â or âHow can one justify a moral belief?â Logic gives tools to deal with such questions. If youâve studied philosophy, youâll likely recognize some of the philosophical reasoning in this book. If you havenât studied philosophy, youâll find this book a good introduction to the subject. In either case, youâll get better at recognizing, understanding, and appraising philosophical reasoning.
Finally, logic can be fun. Logic will challenge your thinking in new ways and will likely fascinate you. Most people find logic enjoyable.
1.2 Valid arguments
I begin my basic logic course with a multiple-choice test. The test has ten problems; each gives information and asks what conclusion necessarily follows. The problems are fairly easy, but most students get about half wrong.2 0002
2 Http://www.harryhiker.com/logic.htm has my pretest in an interactive format. I suggest that you try it. I developed this test to help a psychologist friend test the idea that males are more logical than females; both groups, of course, did equally well on the problems.
Hereâs a problem that almost everyone gets right:
- If you overslept, youâll be late.
- You arenât late.
Therefore
- (a) You did oversleep.
- (b) You didnât oversleep. â correct
- (c) Youâre late.
- (d) None of these follows.
With this next one, many wrongly pick answer â(b)â:
- If you overslept, youâll be late.
- You didnât oversleep.
Therefore
- (a) Youâre late.
- (b) You arenât late.
- (c) You did oversleep.
- (d) None of these follows. â correct
Here âYou arenât lateâ doesnât necessary follow, since you might be late for another reason; maybe your car didnât start.1 The pretest shows that untrained logical intuitions are often unreliable. But logical intuitions can be developed; yours will likely improve as you work through this book. Youâll also learn techniques for testing arguments.
1 These two arguments were taken from Matthew Lipmanâs fifth-grade logic textbook: Harry Stottlemeierâs Discovery (Caldwell, NJ: Universal Diversified Services, 1974).
In logic, an argument is a set of statements consisting of premises (supporting evidence) and a conclusion (based on this evidence). Arguments put reasoning into words. Hereâs an example (ââŽâ is for âthereforeâ):
Valid argument
- If you overslept, youâll be late.
- You arenât late.
- ⎠You didnât oversleep.
An argument is valid if it would be contradictory (impossible) to have the premises all true and conclusion false. âValidâ doesnât say that the premises are true, but only that the conclusion follows from them: if the premises were all true, then the conclusion would have to be true. Here we implicitly assume that thereâs no shift in the meaning or reference of the terms; hence we must use âoverslept,â âlate,â and âyouâ the same way throughout the argument.2
2 Itâs convenient to allow arguments with zero premises; such arguments (like â⎠x = xâ) are valid if and only if the conclusion is a necessary truth (couldnât have been false).
Our argument is valid because of its logical form: how it arranges logical notions like âif-thenâ and content like âYou overslept.â We can display the form using words or symbols for logical notions and letters for content phrases:
- If you overslept, youâll be late.
- You arenât late.
- ⎠You didnât oversleep.
- If A then B Valid
- Not-B
- ⎠Not-A
Our argument is valid because its form is correct. Replacing âAâ and âBâ with other content yields another valid argument of the same form: 0003
- If youâre in France, youâre in Europe.
- You arenât in Europe.
- ⎠You arenât in France.
- If A then B Valid
- Not-B
- ⎠Not-A
Logic studies forms of reasoning. The content can deal with anything â backpacking, math, cooking, physics, ethics, or whatever. When you learn logic, youâre learning tools of reasoning that can be applied to any subject.
Consider our invalid example:
- If you overslept, youâll be late.
- You didnât oversleep.
- ⎠You arenât late.
- If A then B Invalid
- Not-A
- ⎠Not-B
Here the second premise denies the first part of the if-then; this makes it invalid. Intuitively, you might be late for some other reason â just as, in this similar argument, you might be in Europe because youâre in Italy:
- If youâre in France, youâre in Europe.
- You arenât in France.
- ⎠You arenât in Europe.
- If A then B Invalid
- Not-A
- ⎠Not-B
1.3 Sound arguments
Logicians distinguish valid arguments from sound arguments:
An argument is valid if it would be contradictory to have the premises all true and conclusion false.
An argument is sound if itâs valid and every premise is true.
Calling an argument âvalidâ says nothing about whether its premises are true. But calling it âsoundâ says that itâs valid (the conclusion follows from the premises) and has all premises true. Hereâs a sound argument:
Valid and true premises
- If youâre reading this, you arenât illiterate.
- Youâre reading this.
- ⎠You arenât illiterate.
When we try to prove a conclusion, we try to give a sound argument: valid and true premises. With these two things, we have a sound argument and our conclusion has to be true.
An argument could be unsound in either of two ways: (1) it might have a false premise or (2) its conclusion might not follow from the premises: 0004
First premise false
- All logicians are millionaires.
- Gensler is a logician.
- ⎠Gensler is a millionaire.
Conclusion doesnât follow
- All millionaires eat well.
- Gensler eats well.
- ⎠Gensler is a millionaire.
When we criticize an opponentâs argument, we try to show that itâs unsound. We try to show that one of the premises is false or that the conclusion doesnât follow. If the argument has a false premise or is invalid, then our opponent hasnât proved the conclusion. But the conclusion still might be true â and our opponent might later discover a better argument for it. To show a view to be false, we must do more than just refute an argument for it; we must give an argument that shows the view to be false.
Besides asking whether premises are true, we can ask how certain they are, to ourselves or to others. Weâd like our premises to be certain and obvious to everyone. We usually have to settle for less; our premises are often educated guesses or personal convictions. Our arguments are only as strong as their premises. This suggests a third strategy for criticizing an...