John Broadus Watson (1878-1958), one of America's early psychologists who introduced the behaviorist approach to the scientific community, introduced a radical change in the study of psychology whereby he felt that the focus of psychology should be on directly observable behaviors rather than on the study of consciousness as it was then studied.  
He used classical conditioning in order to prove how behaviorism should be the focus of psychology. Watson took psychology out of the realm of subjectivity into the realm of objectivity with his new behaviorist approach that made psychology more scientific and comparable to other scientific studies. 
Nature vs. nurture
Watson's approach to the study of psychology and classical conditoning focused on the nature (genetic inheritance) versus nurture (environment and experience) argument. Man was totally conditioned by his environment. Watson's ideas and study were oriented toward the nurture approach, totally denying the existence of any genetic involvement in a person's development. 
Opposition from the scientific community
There was opposition from the scientific community especially from the Gestalt psychologists who focused highly on perception as being very important to the development of the human being. The gestaltists were in agreement that "conscious experience rather than overt behavior" should be the focus of psychological study. 
Watson's beliefs were considered positive and overly optimistic. He felt that he could weld any infant at random into what he designated, creating a doctor, lawyer, scientist, teacher entirely out of environmental stimuli and response. 
Watson's "view of behaviorism was considered radical and was known for extreme mentalism, its radical reduction of thinking to implicit response, and its heavy and somewhat simplistic reliance on conditioned reactions."
Man as a direct product of his environment
Watson said that "scientists should only study what people do and say, not study thoughts, wishes, and feelings" and that "man is made not born." 
Man was a direct product of his environmental experience. He could therefore be conditioned to respond in artificial ways in controlled experiments or be conditioned by the direct environment that he lived in.
With his famous little Albert experiment where he showed how fear was a conditioned response to negative stimuli, he came up with three emotional responses of children, which were fear, rage, and love. 
Based on these classical conditioning experiments and others he performed on animals, Watson concluded that any sort of genetic hereditary information had no bearing on the conditioning of man. 
Accordingly, man is conditoned by his environment. Man responds and behaves according to outward environmental stimuli. This was the stimulus-response approach or classical conditioning first introduced to the scientific community by Ivan Pavlov, the Russian scientist. Along with Thorndike (1898), John B. Watson (1919) was the foremost contributor to the behaviorist approach in America. 
In conclusion, J. B. Watson used classical conditioning experiments to prove to the scientific community that psychology was a real science without the use of subjectivity. He was the foremost proponent of the behaviorist approach to psychological research and experiment that helped prove psychology was a viable science.
3) Weiten, Wayne, Psychology, 6th ed, Wadsworth, US, 2005