 To make the point in a slightly different way, let me adopt the following as an expository, heuristic device, a harmless fiction. Imagine a partition as carved into a large finite number of equally-probable chunks, such that the propositions with which we are concerned are true in an exact number of them. The probability of any proposition is the proportion of chunks in which it is true. The probability of \(B\) on the supposition that \(A\) is the proportion of the \(A\)-chunks (the chunks in which \(A\) is true) which are \(B\)-chunks. With some misgivings, I succumb to the temptation to call these chunks “worlds”: they are equally probable, mutually incompatible and jointly exhaustive epistemic possibilities, enough of them for the propositions with which we are concerned to be true, or false, at each world. The heuristic value is that judgements of probability and conditional probability then translate into statements about proportions.