Classical Conditioning
Watson believed that all individual differences in behaviour were due to different experiences of learning. He famously said:
Watson proposed that the process of classical conditioning (based on Pavlov’s observations) was able to explain all aspects of human psychology.
Everything from speech to emotional responses were simply patterns of stimulus and response. Watson denied completely the existence of the mind or consciousness.
Watson and Rayner (1920) Little Albert

When "Little Albert" was just over 11 months old the white rat was presented and seconds later the hammer was struck against the steel bar. This was done 7 times over the next 7 weeks and each time "little Albert" burst into tears. By now "little Albert only had to see the rat and he immediately showed every sign of fear. He would cry (whether or not the hammer was hit against the steel bar) and he would attempt to crawl away. Watson and Raynor had shown that classical conditioning could be used to create a phobia. A phobia is an irrational fear, i.e. a fear that is out of proportion to the danger. Over the next few weeks and months "Little Albert" was observed and 10 days after conditioning his fear of the rat was much less marked. This dying out of a learned response is called extinction. However even after a full month it was still evident.