  How can probabilities make sense in a deterministic many-worlds theory? We
address two facets of this problem: why should rational agents assign
subjective probabilities to branching events, and why should branching events
happen with relative frequencies matching their objective probabilities. To
address the first question, we generalise the Deutsch-Wallace theorem to a wide
class of many-world theories, and show that the subjective probabilities are
given by a norm that depends on the dynamics of the theory: the 2-norm in the
usual Many-Worlds interpretation of quantum mechanics, and the 1-norm in a
classical many-worlds theory known as Kent's universe. To address the second
question, we show that if one takes the objective probability of an event to be
the proportion of worlds in which this event is realised, then in most worlds
the relative frequencies will approximate well the objective probabilities.
This suggests that the task of determining the objective probabilities in a
many-worlds theory reduces to the task of determining how to assign a measure
to the worlds.
