John B. Watson
The 20th century saw a reaction to Edward Titchener's critique of Wundt's empiricism. This contributed to the formulation of behaviorism by John B. Watson, which was popularized by B. F. Skinner. Behaviorism proposed emphasizing the study of overt behavior, because that could be quantified and easily measured. Early behaviorists considered study of the "mind" too vague for productive scientific study. However, Skinner and his colleagues did study thinking as a form of covert behavior to which they could apply the same principles as overt (publicly observable) behavior.