  The aim of this paper is to propose an alternative behavioural definition of
computation (and of a computer) based simply on whether a system is capable of
reacting to the environment-the input-as reflected in a measure of
programmability. This definition is intended to have relevance beyond the realm
of digital computers, particularly vis-a-vis natural systems. This will be done
by using an extension of a phase transition coefficient previously defined in
an attempt to characterise the dynamical behaviour of cellular automata and
other systems. The transition coefficient measures the sensitivity of a system
to external stimuli, and will be used to define the susceptibility of a system
to being (efficiently) programmed.
