Valid and Invalid Arguments Lecture 3 Section 1
- Slides: 24
Valid and Invalid Arguments Lecture 3 Section 1. 3 Mon, Jan 23, 2006
Arguments An argument is a sequence of statements. ¢ The last statement is the conclusion. ¢ All the other statements are the premises. ¢ A mathematical proof is an argument. ¢
Argument Forms An argument form is a sequence of statement forms. ¢ The last statement form is the conclusion. ¢ All the other statement forms are the premises. ¢ A mathematical proof follows an argument form. ¢
Validity of an Argument Form An argument form is valid if its conclusion is true when its premises are true. ¢ Otherwise, the argument form is invalid. ¢ An invalid argument form is called a fallacy. ¢
Validity of an Argument ¢ An argument is valid if its argument form is valid, whether or not its premises are true.
The Form of an Argument Let the premises be P 1, …, Pn. ¢ Let the conclusion be C. ¢ The argument form is valid if P 1 Pn C is a tautology. ¢
Example I will ride my bike today. ¢ If it is raining and I ride my bike, then I will get wet. ¢ It is raining. ¢ Therefore, I will get wet. ¢
Example ¢ ¢ p = “I will ride my bike today. ” q = “It is raining. ” r = “I will get wet. ” Argument form: p q p r q r
Example P 1 P 2 P 3 C P 1 P 2 P 3 C p q p r q r T T T F T T F F T T T T F T F T T F F T
Example: Invalid Argument Forms with True Conclusions ¢ An argument form may be invalid even though its conclusion is true. If I eat my vegetables, I’ll be big and strong. l I’m big and strong. l Therefore, I ate my vegetables. l ¢ A true conclusion does not ensure that the argument form is valid.
Example: Valid Argument Forms with False Conclusions ¢ An argument form may be valid even though its conclusion is false. If I wait until the last minute to do my homework, then it will be a lot easier. l I wait until the last minute to do my homework. l Therefore, it will be a lot easier. l ¢ A false conclusion does not mean that the argument form is invalid.
Example: Valid Argument Forms with False Conclusions ¢ Another example. If 1 + 1 = 2, then pigs can fly. l 1 + 1 = 2. l Therefore, pigs can fly. l
Modus Ponens Modus ponens is the argument form p q ¢ This is also called a direct argument. ¢
Examples of Modus Ponens If it is raining, then I am carrying my umbrella. It is raining. Therefore, I am carrying my umbrella. ¢ If pigs can fly, then I am carrying my umbrella. Pigs can fly. Therefore, I am carrying my umbrella. ¢
Modus Tollens Modus tollens is the argument form p q q p ¢ This is also called an indirect argument. ¢ It is equivalent to replacing p q with q p and then using modus ponens. ¢
Examples of Modus Tollens If it is raining, then I am carrying my umbrella. I am not carrying my umbrella. Therefore, it is not raining. ¢ If pigs can fly, then I am carrying my umbrella. I am not carrying my umbrella. Therefore, pigs cannot fly. ¢
Other Argument Forms From the specific to the general p p q ¢ From the general to the specific p q p ¢
Other Argument Forms Elimination p q ¢ Transitivity p q q r p r ¢
Other Argument Forms ¢ Division into Cases p q p r q r r
Fallacies A fallacy is an invalid argument form. ¢ Two common fallacies ¢ The fallacy of the converse. l The fallacy of the inverse. l
The Fallacy of the Converse The fallacy of the converse is the invalid argument form p q q p ¢ This is also called the fallacy of affirming the consequent. ¢
Example ¢ If it is raining, then I am carrying an umbrella. Therefore, it is raining.
Fallacy of the Inverse The fallacy of the inverse is the invalid argument form p q p q ¢ This is also called the fallacy of denying the antecedent. ¢
Example ¢ If pigs can fly, then I am carrying an umbrella. Pigs cannot fly. Therefore, I am not carrying an umbrella.
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