  We make a critical review of the semiclassical interpretation of quantum
cosmology and emphasise that it is not necessary to consider that a concept of
time emerges only when the gravitational field is (semi)classical. We show that
the usual results of the semiclassical interpretation, and its generalisation
known as the Born-Oppenheimer approach to quantum cosmology, can be obtained by
gauge fixing, both at the classical and quantum levels. By `gauge fixing' we
mean a particular choice of the time coordinate, which determines the arbitrary
Lagrange multiplier that appears in Hamilton's equations. In the quantum
theory, we adopt a tentative definition of the (Klein-Gordon) inner product,
which is positive definite for solutions of the quantum constraint equation
found via an iterative procedure that corresponds to a weak coupling expansion
in powers of the inverse Planck mass. We conclude that the wave function should
be interpreted as a state vector for both gravitational and matter degrees of
freedom, the dynamics of which is unitary with respect to the chosen inner
product and time variable.
