Certainty and Impossibility

19 August 2009

Recently, when working on my next paper, I was struck by the formal similarity between two expressions.

Using (X|c) to represent outcome X of action c, and ~ to represent a relationship of equal plausibility, [(X_i v X_j | c) ~ (X_i | c)] for_all X_j implies that Xi is certain given c, and [(X_i v X_j | c) ~ (X_j | c)] for_all X_j implies that Xi is impossible given c. (The two expressions differ in the subscript of the outcome on the right-hand side of the relation.)

The ideas here are simple: If no other outcome contributes plausibility, then Xi is certain; if Xi contributes no plausibility, then Xi is impossible.

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