For hundreds of years philosophers speculated about "the mind" and in around the 1880's the popular method of psychology dealt only with the conscious mind. The experiments carried out at this time were criticised for their lack of objectivity and by the 1920's a new brand of psychology emerged in the form of behaviourism.
Psychology became a recognised discipline in around 1897 when Wilhelm Wundt started the first psychology lab in Germany. Wundt, along with others, attempted to investigate the mind through introspection, and observed their own conscious mental processes. While analysing their thoughts, images and feelings, they recorded and measured their results under controlled conditions and aimed to sort conscious thought into its basic elements as a chemist would with a chemical compound. This theory was known as structuralism.
A particular critic of this method, in the early 1920's was John Broadus Watson (1878-1958), who felt that introspection was subjective and therefore erroneous.
He also felt the only way forward was by using methods that could be observed by more that just one person and this could be achieved by studying behaviour. He wrote that "Behaviourism claims that 'consciousness' is neither a definable nor a usable concept; that it is merely another word for the 'soul' of more ancient times." (Watson 1924)
Behaviourist theories of learning are often called "stimulus-response" (S-R), and though only classical conditioning fits the S-R model, the other major form, operant conditioning, is often included under the same heading, though it is significantly different. Classical conditioning is triggered involuntarily by a particular environmental stimulus. This means that a stimulus that does not normally produce a particular response can be paired with another stimulus that does, eventually resulting in both stimuli inducing the same effect, even when used separately.
A good example of this was shown...