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In philosophy and logic, contingency is the status of propositions that are neither true under every possible valuation (i.e. tautologies) nor not false under every possible valuation (i.e. contradictions). A contingent proposition is not necessarily true or necessarily false. Propositions that are contingent may be so because they contain logical connectives which, along with the truth value of any of its atomic parts, determine the truth value of the proposition. This is to say that the truth value of the proposition is contingent upon the truth values of the sentences which comprise it. Contingent propositions depend on the facts, whereas analytic propositions are true without regard to any facts about which they speak.
Along with contingent propositions, there are at least three other classes of propositions, some of which overlap:
- Tautological propositions, which must be true, no matter what the circumstances are or could be (example: "The sky is blue or the sky is not blue.").
- Contradictions which must necessarily be untrue, no matter what the circumstances are or could be (example: "It's raining and it's not raining.").
- 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.
- Michael Shermer, "Glorious Contingency," Metanexus Net 
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