Cognitive science has replaced behaviourism as the dominant force within contemporary psychology. Psychology is purported to be a rational science, yet this most recent paradigmatic shift cannot be rationally explained or justified. Modern and sophisticated behavioural theories meet the accepted criteria for good explanatory theory within a rational science, while modern cognitive theories do not. Sophisticated forms of behavioural theory exhibit explanatory power and plausibility. In contrast, all cognitive theories are based upon a set of fundamentally flawed assumptions that preclude the development of plausible psychological explanation. In spite of claims to the contrary, modern cognitive theories continue to exhibit these logical flaws. Moreover, any plausible interpretations or aspects of modern cognitive theories are those compatible with the assumptions of behaviourism and devoid of cognitive explanatory constructs. Cognitive psychology is based upon assumptions of rationality, yet its current dominance cannot be rationally justified. Behaviourism rejects the premise of rational action, yet is acceptable according to the criteria of a rational science. Such a state of affairs can only be explained from within a behavioural framework.