Partly in reaction to the subjective and introspective nature of Freudian psychodynamics, and its focus on the recollection of childhood experiences, during the early decades of the 20th century, behaviorism gained popularity as a guiding psychological theory. Founded by John B. Watson and embraced and extended by Edward Thorndike, Clark L. Hull, Edward C. Tolman, and later B.F. Skinner), behaviorism was grounded in studies of animal behavior. Behaviorists shared the veiw that the subject matter of psychology should be operationalized with standardized procedures which led psychology to focus on behavior, not the mind or consciousness. They doubted the validity of introspection for studying internal mental states such as feelings, sensations, beliefs, desires, and other unobservables. in "Psychology as the Behaviorist Views It" (1913), Watson argued that psychology "is a purely objective experimental branch of natural science," that "introspection forms no essential part of its methods", and that "the behaviorist recognizes no dividing line between man and brute." Skinner rejected hypothesis testing as a productive method of research, considering it to be too conducive to speculative theories that would promote useless research and stifle good research.