In psychology as the behaviorist views it, watson argues that psychology is a radical environmentalism in kuhn's terms, a "paradigm" a purely experimental branch of natural science a return to descartes's dualism big enough to include both mental and behavioral phenomena

I believe the answer is: a purely experimental branch of natural science

According to watson, every act of treatments that is aimed to fix a certain psychological problems is only based on prediction since there is no sure thing to predict other people's behavior due to the number of factors that might influence it. Because of this, he refers to psychology as a 'purely experimental;' branch.