Many psychologists at the time, influenced especially by Sigmund Freud, were interested in the innate and internal workings of the mind as the sources of human behavior. Watson, on the other hand, was an early behaviorist, and argued that human behavior was largely the product of external stimuli. Rather than studying the mind as an abstract thing, it was better to study how people's behaviors responded to different methods of conditioning. As he claimed in his book Behavior: An Introduction to Comparative Psychology, the approach which focused on the internal workings of the mind was outdated:
The time seems to have come when psychology must discard all reference to consciousness; when it need no longer delude itself into thinking that it is making mental states the subject of observation...It is possible...to write a psychology, to define it...as the "science of behavior."
This led Watson, who studied both people and animals, and was struck by the similarities between them, to argue that the mind was best studied through behavior. While certain behaviors were obviously innate, they became conditioned very shortly after birth by external stimuli. Babies cried, grasped, and even blinked and hiccupped in ways that were conditioned by the mother's responses to them. Watson, who became regarded as an expert in child-rearing, emphasized that fear of such things as the dark was also not related to any instinct, but rather was the product of conditioned behavior.