John B. Watson.
Behaviorism’s strongest advocate was John B. Watson. He suggested that behavior was a better focus for psychology, because behavior is objective not subjective. Also, Watson wanted to achieve universal psychological laws. Laws that included animals. Animals cannot introspect, or at least we cannot collect their introspective reports, so the introspection method excludes them from psychology. But by focusing on behavior, psychology can include animals as subjects of study, because animal behavior can be studied just as well as human behavior. Furthermore, as we are determining the universal laws of behavior, what we discover in studying animals should be applicable to humans. Finally, Watson was also displeased about introspective reporters requiring training. He believed that such training could lead to biased introspection reports.