The second law states that any closed system follows the arrow of time, meaning its entropy never decreases.
This idea has been discussed since the development of statistical mechanics,[Note 1] but the term "past hypothesis" was coined by philosopher David Albert in 2000.
[4] The past hypothesis is an exception to the principle of indifference, according to which every possible microstate within a certain macrostate would have an equal probability.
If the principle of indifference is applied without taking into account the past hypothesis, a low- or medium-entropy state would have likely evolved both from and toward higher-entropy macrostates, as there are more ways statistically to be high-entropy than low-entropy.
[5] Common theoretical frameworks have been developed in order to explain the origin of the past hypothesis based on inflationary models or the anthropic principle.