  A model of the evolution of cognition is used to derive a Requirement
Equation (RE), which defines what computations the fittest possible brain must
make, or must choose actions as if it had made those computations. The terms in
the RE depend on factors outside an animals brain, which can be modelled
without making assumptions about how the brain works, from knowledge of the
animals habitat and biology. In simple domains where the choices of actions
have small information content, it may not be necessary to build internal
models of reality; short cut computations may be just as good at choosing
actions. In complex domains such as 3D spatial cognition, which underpins many
complex choices of action, the RE implies that brains build Bayesian internal
models of the animals surroundings; and that the models are constrained to be
true to external reality.
