1. Disjunctive Syllogism
Either/Or Statements

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

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

4. 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.

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

6. 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)

7. 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)

8. 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.