Behaviourism - Eric Kandel

Behaviorists, notably J. B. Watson and B. F. Skinner, argued that behaviour could be studied with the same precision achieved in the physical sciences but only if students of behaviour abandoned speculation about what goes on in the mind (the brain) and focused instead on observable aspects of behaviour.

For behaviorists, unobservable mental processes, specially anything as abstract as conscious awareness was deemed inaccessible to scientific study ... Their early successes in rigorously studying simple forms of behaviour and learning encouraged them to consider irrelevant to a scientific study of behavior the processes that intervene between stimulus and behavior.