Equipossible

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Equipossibility is a philosophical concept in possibility theory that is a precursor to the notion of equiprobability in probability theory. It is used to distinguish what can occur in a probability experiment. For example, when considering rolling a six-sided die, why do we typically view the possible outcomes as {1,2,3,4,5,6} rather than, say, {6, not-6}? The former set contains equally possible alternatives, while the latter does not because there are five times as many alternatives inherent in not-6 as in 6. This is true even if the die is biased so that 6 and not-6 are equally likely to occur.

By the Principle of Indifference of Laplace, equipossible alternatives may be accorded equal probabilities if nothing more is known about the underlying probability distribution.

It is a matter of contention whether the concept of equipossibility, also called equispecificity (from equispecific), can truly be distinguished from the concept of equiprobability.