In psychology, mentalism is an umbrella term that refers to those branches of study that concentrate on mental perception and thought processes, in other words, cognition, like cognitive psychology. This is in opposition to disciplines, most notably behaviorism, that believe the study of psychology should focus on the structure of causal relationships to conditioned responses, that is to say behaviors, and seek to support this hypothesis through scientific methods and experimentation. Over the course of the history of psychology, mentalism and behaviorism have clashed, with one or the other representing the dominant paradigm of psychological investigation at different times in history.
It is worth noting that neither mentalism nor behaviorism are mutually exclusive fields; elements of one can be seen in the other, perhaps moreso in modern times compared to the advent of psychology over a century ago.
Classical mentalism 
Mentalism dates back to the very founding of the field of psychology. "Classical Mentalism", as it is sometimes called, tied together many differing schools of psychological thought from the beginning, and introspective techniques were the norm when it came to research, making psychology an inherently subjective field. Prominent figures ranged from Edward Titchener to William James; despite Titchener being a Structuralist and James being of the Functionalist school of thought, both agreed on one thing: consciousness was indisputably the subject matter of psychology, making them both Mentalists.
Behaviorism takes over 
Concurrently thriving alongside mentalism since the inception of psychology was the perspective of behaviorism. However, it was not until 1913, when psychologist John B. Watson published his article "Psychology as the Behaviorist Views It" that behaviorism began to steal the spotlight from classical mentalism. Watson's ideas sparked what some have called a paradigm shift in the world of psychology, leading to the objective and experimental study of human behavior, rather than subjective, introspective study of human consciousness—the study of which was seen as impossible to truly do, and the focus on it to that point had only been a hindrance to the field reaching its full potential. For a time, behaviorism would go on to be the dominant force driving psychological thought, advanced by the work of other luminaries such as Ivan Pavlov, Edward Thorndike, and B.F. Skinner. Of course, mentalism did not simply die; much like how behaviorism had coexisted beside mentalism earlier in history, so too did mentalism continue to exist, just not as the current ruling theory of scientific psychological thought.
The new mentalism 
The reign of Behaviorism was not to last, however. While it remains a thriving, vibrant field to this day, a scathing review of B.F. Skinner's "Verbal Behavior" by Noam Chomsky in 1959 heralded a shift back to a focus on consciousness in psychology with the beginning of the cognitive revolution. Critical to the successful revival of the mind or consciousness as the primary focus of study in psychological inquiry were advances in the computer sciences and neurosciences, which allowed for actual brain mapping, among other things. At last, mentalism had an objectively experimental way to begin to study the mind, effectively nullifying the main criticism that led to its downfall half a century earlier.
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