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Sociology 220

Logic: Syllogisms. A syllogism is a three-line logical form.The first two lines are propositions (or premises)The last line draws a conclusion from those premises. Why bother studying syllogisms?. Certain kinds of syllogisms give logically true results:If A then BIf B then CThe

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Sociology 220

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    1. Sociology 220 January 28, 2009 Prof. Brines MGH 231

    2. Logic: Syllogisms A syllogism is a three-line logical form. The first two lines are propositions (or premises) The last line draws a conclusion from those premises

    3. Why bother studying syllogisms? Certain kinds of syllogisms give logically true results: If A then B If B then C Therefore: If A then C Some combinations of propositions give logically incorrect or flawed results. It’s important to be able to recognize these when they’re being used.

    4. Propositional Forms Categorical: “A leads to B” “Democracies promote free markets.” Conditional: “If A then B” “If you earn a college degree, (then) you will get a better job.”

    5. Valid forms of reasoning

    6. Deductive Reasoning reasoning which uses deductive arguments to move from given statements (propositions) to conclusions, which must be true if the propositions are true. Here, syllogism = argument

    7. Modus Ponens (affirming the antecedent) If A, then B A Therefore: B If I am a senior, I have to register today. I am a senior Therefore: I have to register today

    8. Modus Tollens (denying the consequent) If A, then B Not B Therefore: Not A If the US is a monarchy, then it has a king. The US does not have a king. Therefore: The US is not a monarchy.

    9. Invalid Deductive Inferences

    10. Denying the Antecedent If A, then B Not A *Therefore: Not B If someone is a British subject, then he is a human being Bill Gates is not a British subject *Therefore: Bill Gates is not a human being

    11. Affirming the Consequent If A then B B *Therefore: A If I am a freshman, I have to register today I have to register today *Therefore: I am a freshman (Wrong!)

    12. Examples If I have the flu, then I have a sore throat I have a sore throat Therefore: I have the flu True or false? Type of syllogism?

    13. If 25 people sign up, there will be a party 25 people have not signed up Therefore: There will not be a party. True or false? Type of syllogism?

    14. If Lizzie is the murderer, then she owns an axe. Lizzie does not own an axe. Therefore: Lizzie is not the murderer. True or False? Type of syllogism?

    15. If an intruder is detected, the alarm goes off. The alarm does not go off. Therefore, no intruder is detected. True or False? Type of syllogism?

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