  Assuming that quantum states, including pure states, represent subjective
degrees of belief rather than objective properties of systems, the question of
what other elements of the quantum formalism must also be taken as subjective
is addressed. In particular, we ask this of the dynamical aspects of the
formalism, such as Hamiltonians and unitary operators. Whilst some operations,
such as the update maps corresponding to a complete projective measurement,
must be subjective, the situation is not so clear in other cases. Here, it is
argued that all trace preserving completely positive maps, including unitary
operators, should be regarded as subjective, in the same sense as a classical
conditional probability distribution. The argument is based on a reworking of
the Choi-Jamiolkowski isomorphism in terms of "conditional" density operators
and trace preserving completely positive maps, which mimics the relationship
between conditional probabilities and stochastic maps in classical probability.
