Contingency (philosophy)

In common usage a contingency is a conditional response plan made in preparation for various future circumstances including the unanticipated.
In philosophy and logic, contingency is the status of propositions that are not necessarily true or necessarily false. Here are four classes of propositions, some of which overlap:

• necessarily true propositions, which must be true, no matter what the circumstances are or could be (examples: 2 + 0 = 2; All bachelors are unmarried).

• necessarily false propositions, which must be false, no matter what the circumstances are or could be (examples: 2 + 2 = 5; Anne is both taller than and shorter than Brad).

• contingent propositions, which are not necessarily true and not necessarily false (examples: There are only three planets; There are more than three planets).

• possible propositions, which are true or could have been true given certain circumstances (examples: x + y = 4; There are only three planets; There are more than three planets). All necessarily true propositions, and all contingent propositions, are also possible propositions.

Usually, necessary proposition is understood to mean necessarily true proposition.