Functional psychology or functionalism refers to a psychological philosophy that considers mental life and behaviour in terms of active adaptation to the person's environment. As such, it provides the general basis for developing psychological theories not readily testable by controlled experiments and for applied psychology.
Functionalism arose in the U.S. in the late 19th century as an alternative to structuralism. While functionalism never became a formal school, it built on structuralism's concern for the anatomy of the mind and led to greater concern over the functions of the mind, and later to behaviourism.
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Functionalism  was a philosophy opposing the prevailing structuralism of psychology of the late 19th century. Edward Titchener, the main structuralist, gave psychology its first definition as a science of the study of mental experience, of consciousness, to be studied by trained introspection.
William James is considered to be the founder of functional psychology. Although he would not consider himself as a functionalist, nor did he truly like the way science divided itself into schools. John Dewey, George Herbert Mead, Harvey A. Carr, and especially James Rowland Angell were the main proponents of functionalism at the University of Chicago. Another group at Columbia, including notably James McKeen Cattell, Edward L. Thorndike, and Robert S. Woodworth, were also considered functionalists and shared some of the opinions of Chicago's professors. Egon Brunswik represents a more recent, but Continental, version. The functionalists retained an emphasis on conscious experience.
Behaviourists also rejected the method of introspection but criticized functionalism because it was not based on controlled experiments and its theories provided little predictive ability.B.F. Skinner was a developer of behaviourism. He did not think that considering how the mind affects behaviour was worthwhile, for he considered behaviour simply as a learned response to an external stimulus. Yet, such behaviourist concepts tend to deny the human capacity for random, unpredictable, sentient decision-making, further blocking the functionalist concept that human behaviour is an active process driven by the individual. Perhaps, a combination of both the functionalist and behaviourist perspectives provides scientists with the most empirical value, but, even so, it remains philosophically (and physiologically) difficult to integrate the two concepts without raising further questions about human behaviour. For instance, consider the interrelationship between three elements: the human environment, the human autonomic nervous system (our fight or flight muscle responses), and the human somatic nervous system (our voluntary muscle control). The behaviourist perspective explains a mixture of both types of muscle behaviour, whereas the functionalist perspective resides mostly in the somatic nervous system. It can be argued that all behavioural origins begin within the nervous system, prompting all scientists of human behaviour to possess basic physiological understandings, something very well understood by the functionalist founder William James.
Evolutionary psychology is based on the idea that knowledge concerning the function of the psychological phenomena affecting human evolution is necessary for a complete understanding of the human psyche. Even the project of studying the evolutionary functions of consciousness is now an active topic of study. Like evolutionary psychology, James's functionalism was inspired by Charles Darwin's theory of natural selection.