Arguments Arguments: premises provide grounds for the truth of the conclusion Two different ways a conclusion may be supported by premises. Deductive Arguments.

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Presentation transcript:

Arguments Arguments: premises provide grounds for the truth of the conclusion Two different ways a conclusion may be supported by premises. Deductive Arguments Inductive Arguments

Deductive Arguments Aim: logical necessity; the truth of the conclusion follows necessarily from the truth of the premises. Succeeds: Valid Fails: Invalid

Validity Valid Argument: the truth of the premises guarantees the truth of the conclusion; if the premises are true the conclusion must be true If Socrates is a philosopher then he isn’t a ballerina. Socrates is a philosopher. So, Socrates isn’t a ballerina.

Invalidity Invalid Argument: the truth of the premises does not guarantee the truth of the conclusion; it’s possible for the conclusion to be false if the premises are true. If Socrates is a philosopher then he isn’t a ballerina. Socrates isn’t a ballerina. So, Socrates is a philosopher.

Validity & Truth Validity: Arguments Truth: Propositions Every argument makes a claim about the relation between its premises and the conclusion Truth or falsity of conclusion doesn’t determine its validity or invalidity; the logical form of the relation does that

Valid Argument w/False Conclusion If Frazier keeps Ali from hitting him in the left eye, he wins the Thrilla in Manilla. Frazier keeps Ali from hitting him in the left eye. Therefore, Frazier wins the Thrilla in Manila.

Invalid Argument w/True Conclusion If I broke the bank at Monte Carlo, then I would be wealthy. I didn’t break the bank at Monte Carlo. Therefore, I’m not wealthy.

Disjunctive Syllogism Either you love me or you leave me. It’s not the case that you love me. Therefore, you leave me.

Disjunctive Syllogism Either human beings want peace or war. Clearly history shows they don’t want peace. Therefore, they want war.

Conditional Material Implication Implication: the relation that holds between the antecedent and the consequent of a conditional. There are different kinds of implication.

Logical Implication Consequent follows logically from antecedent. If all humans are mortal and Socrates is a human, then Socrates is mortal.

Definitional Implication Consequent follows from its antecedent by the definition of the term. If Johnny is a bachelor, then Johnny is unmarried.

Causal Implication Consequent follows causally from its antecedent. If this piece of blue litmus paper is placed in acid, then this piece of blue litmus paper will turn red.

Decisional Implication Consequent follows from the antecedent given a decision one makes. If my sister asks me one more time when am I going to give her a little niece or nephew, I’m slitting my wrists.

Modus Ponens If my sister asks me one more time when am I going to give her a little niece or nephew I ’ m slitting my wrists. My sister asks me one more time when am I going to give her a little niece or nephew. So, I ’ m slitting my wrists.

Modus Tollens If Sarah Palin becomes president then I ’ m moving to the Negev desert to live amongst the Bedouin. I am not moving to the Negev desert to live amongst the Bedouin. Therefore, Sarah Palin doesn ’ t become president.

If people are entirely rational then all of a person’s actions can be predicted in advance. Not all of a person’s actions can be predicted in advance. Thus, people are not entirely rational.

Hypothetical Syllogism If Sarah Palin becomes president then I ’ m moving to the Negev desert to live amongst the Bedouin. If I move to the Negev desert to live amongst the Bedouin then I ’ ll be certain to tie my camel. If Sarah Palin becomes president then I ’ ll be certain to tie my camel.

If people are entirely rational then all of a person’s actions can be predicted in advance. If all of a persons’ actions can be predicted in advance then the universe is essentially deterministic. So if people are entirely rational then the universe is essentially deterministic.

Affirming the Consequent If my sister asks me one more time when am I going to give her a little niece or nephew, I ’ m slitting my wrists. I ’ m slitting my wrists. So, my sister asks me one more time when am I going to give her a little niece or nephew.

Denying the Antecedent If Sarah Palin becomes president then I ’ m moving to the Negev desert to live amongst the Bedouin. Sarah Palin doesn ’ t become president. Therefore, I ’ m not moving to the Negev desert to live amongst the Bedouin.