Disjunctive syllogism

Disciplines > Argument > Syllogisms > Disjunctive syllogism

Description | Example | Discussion | See also

Description

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.

Major premise

The major premise is given in the form of a choice between alternative. The choice is that one out of two or more alternatives is right and that the rest are wrong.

It may appear in a single sentence:

Either Jim, Fred or Billy did it.

Minor premise

The minor premise either selects or rejects alternatives, thus leading to the conclusion.

Jim was in the bar. But Fred had the motive.

Conclusion

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.

Example

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.

Discussion

When comparing two or more items, you are using the contrast principle to use each one to highlight the differences between it and the other.

A fallacy that happens here is 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. Thus if one has a particular characteristic, the other is assumed not to have any of this characteristic. Thus you can cast yourself and your ideas as good by criticizing others as bad. The other guy is bad, which means I am good.