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Roughly speaking, we are told that at least one of two statements is true; then we are told that it is not the former that is true; so we infer that it has to be the latter that is true. First, it is a syllogism--a three-step argument--and second, it contains a disjunction, which means simply an "or" statement. Here is an example: Either I will choose soup or I will choose salad. I will not choose soup. Therefore, I will choose salad.
What is Disjunctive Syllogism