 There are three immediate questions regarding this proposal. The first is called the “reference class” problem. The assignments of probabilities to events requires specifying how they are grouped  together.[46]  Obviously, what is typical with respect to one reference class will not be typical with respect to another (compare, for example, “conscious observers” with “carbon-based life”). Second, the principle of indifference has been thoroughly criticized as a justification for probability in other contexts; what justifies the use of indifference in this case? Why should we take ourselves as “randomly chosen” among an appropriate reference class? The third problem reflects the intended application of these ideas: Bostrom and other authors in this line of work are particularly concerned with observes that may occupy an infinite universe. There is no proof that the universe is in fact infinite. These are all pressing problems for those who hold that the principle of indifference is essential to making cosmological predictions.