Disjunctive Syllogism

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Presentation transcript:

Disjunctive Syllogism Either/Or Statements

Parts Major Premise: presents an “either/or” statement” with two alternatives Minor Premise: Denies or affirms one of the alternatives Conclusion

Valid Forms Minor Premise: Denies one of the alternatives presented in major. Conclusion: Affirms the other alternative.

Valid example Bill will either pass Critical Thinking or he will fail. Bill did not pass Critical Thinking, so he failed. Major Premise: Bill will either pass Critical Thinking (ALT #1) or he will fail (ALT #2). Minor Premise: Bill did not pass Critical Thinking (Denies Alt. #1) Conclusion: Bill failed.

Invalid Form Minor premise affirms one alternative offered in major premise Conclusion denies the remaining alternative.

Fallacy A false disjunction (fallacy) occurs when the major premise limits the alternatives to only two, when further investigation indicates that other alternatives are still possible. Major Premise: WHS students will either go to a university or a tech college upon graduation. Minor Premise: That student will not go to a university. (denies one alternative) Conclusion: That student will attend technical college. (affirms the other alternative)

Invalid Example Either you are for me 100% on this issue or you are against me and what I stand for. You are for me and what I stand for, so you are not against me on this issue. Major Premise: Either you are for me 100% on this issue, or you are against me and what I stand for. Minor Premise: You are for me and what I stand for. (affirms one alternative) Conclusion: You are not against me on this issue. (denies the other alternative)

The conclusion of a disjunctive syllogism can be accepted as truth if the argument satisfies the following conditions: 1) the minor premise correctly limits the two alternatives and 2) the argument is presented in a valid form.