Abstract
This paper questions the adequacy of traditional models of human behaviour, including models of functional mental illness. It is proposed that random (chance) variation is an invariable rule of nature which must apply to neurophysiological processes. Because of this, internal chance (random variation in neurophysiological processes) must be an important factor in determining the course and onset of mental illness. It is proposed that external chance - fortuitous changes in the complex pattern of interaction between physical and functional factors influencing behaviour - is also an important determinant of mental illness. It is proposed that many of the key characteristics of mental illness are explicable in terms of the properties of complex interactional systems.