At the 1916 meeting of the American Psychological Association in New York City, Raymond Dodge delivered the presidential address entitled "The Laws of Relative Fatigue." His words set the tone for the historical background of this thesis. For students of the present generation, Dodge's statement may seem very puzzling. What is this all-important and pervasive phenomenon which he does not even feel needs a definition? "The concept of mental fatigue is so familiar that a precise analysis of its differentia has seldom seemed necessary" (p. 93)