The unity of psychology as a science is to be found in its definition as the science of mental life, and its explanation of individual behavior in terms of mental states. This disciplinary focus will help negotiate psychology's relations with other disciplines, such as neuroscience and cognitive science. The unity within psychology between science and practice is to be found in a focus on scientific evidence as the source of the status, autonomy, and privileges of professional practitioners. Psychology should avoid the temptations of reductionism, and assert (and enjoy) its twin status as both a biological science and a social science. © 2004 Wiley Periodicals, Inc. J Clin Psychol.