  The cognitive state of mind concerning a range of choices to be made can
effectively be modelled in terms of an element of a high-dimensional Hilbert
space. The dynamics of the state of mind resulting form information acquisition
is characterised by the von Neumann-L\"uders projection postulate of quantum
theory. This is shown to give rise to an uncertainty-minimising dynamical
behaviour equivalent to the Bayesian updating, hence providing an alternative
approach to characterising the dynamics of cognitive state that is consistent
with the free energy principle in brain science. The quantum formalism however
goes beyond the range of applicability of classical reasoning in explaining
cognitive behaviours, thus opens up new and intriguing possibilities.
