Conventional statistical mechanics, in other words, makes a crucial assumption: Given that we know we are in a certain macrostate, and that we understand the complete set of microstates corresponding to that macrostate, we can assume that all such microstates are equally likely. We can’t avoid invoking some assumption along these lines; otherwise there’s no way of making the leap from counting states to assigning probabilities. The equal-likelihood assumption has a name that makes it sound like a dating strategy for people who prefer to play hard to get: the “Principle of Indifference.”
