  Probabilities may be subjective or objective; we are concerned with both
kinds of probability, and the relationship between them. The fundamental theory
of objective probability is quantum mechanics: it is argued that neither Bohr's
Copenhagen interpretation, nor the pilot-wave theory, nor stochastic
state-reduction theories, give a satisfactory answer to the question of what
objective probabilities are in quantum mechanics, or why they should satisfy
the Born rule; nor do they give any reason why subjective probabilities should
track objective ones.
  But it is shown that if probability only arises with decoherence, then they
must be given by the Born rule. That further, on the Everett interpretation, we
have a clear statement of what probabilities are, in terms of purely
categorical physical properties; and finally, along lines recently laid out by
Deutsch and Wallace, that there is a clear basis in the axioms of decision
theory as to why subjective probabilities should track these objective ones.
  These results hinge critically on the absence of hidden-variables or any
other mechanism (such as state-reduction) from the physical interpretation of
the theory.
  The account of probability has traditionally been considered the principal
weakness of the Everett interpretation; on the contrary it emerges as one of
its principal strengths.
