equiprobable having the same probability. Sometimes used in the same way as ‘equipossible’, the term is associated with Laplace’s (the ‘classical’) interpretation of probability, where the probability of an event is the ratio of the number of equipossibilities favorable to the event to the total number of equipossibilities. For example, the probability of rolling an even number with a ‘fair’ six-sided die is ½ – there being three equipossibilities (2, 4, 6) favorable to even, and six equipossibilities (1, 2, 3, 4, 5, 6) in all (and without altering the meaning of that context. In truth-functional logic, two statements are logically equivalent if they can never have truthvalues different from each other. In this sense of ‘logically equivalent’ all tautologies are equivalent to each other and all contradictions are equivalent to each other. Similarly, in extensional set theory, two classes are equivalent provided they have the same numbers, so that all empty classes are regarded as equivalent. In a non-extensional set theory, classes would be equivalent only if their conditions of membership were logically equivalent or equivalent in meaning. R.P.