How we change what others think, feel, believe and do
The basic form of the disjunctive syllogism is: Either A is true or B is true. (A exclusive-or B). Thus, if A is true, B is false, and if B is true, A is false. A and B cannot both by true.
The major premise is given in the form of a choice between alternatives, with the assumption that one out of two or more alternative choices is right and that the rest are wrong.
This may appear in a single sentence:
Either Jim, Fred or Billy did it.
The minor premise either selects or rejects alternatives, thus leading to the conclusion.
Jim was in the bar. But Fred had the motive.
The conclusion may be spoken, although often it is not, as it is intended that the target of the major premise concludes this by his or herself. For example:
Fred killed Julius.
Politicians love disjunctive syllogisms, as they offer stark choices:
Either you vote for me or you vote for disaster.
Advertisers love them too. Note here how an airline uses unspoken scare tactics about driving or going by train.
Flying is the safest way to travel.
When comparing two or more items, you are using the contrast principle in highlighting the differences between a target item and the other items.
A fallacy happens here when it is assumed that the choices offered are the only choices. By offering alternatives, the listener is given the impression that this is all there is, and that other choices do not exist. This is the basis of the sales person's alternative close.
Another fallacy occurs where it is assumed that the two alternatives are mutually exclusive. So if one has a particular characteristic, the other is assumed not to have any of this characteristic. For example, you can cast yourself and your ideas as good by criticizing others as bad. The other guy is bad, which means I am good.