Propaganda is not a science in the laboratory sense, but it is no longer entirely the empirical affair that it was before the advent of the study of mass psychology. It is now scientific in the sense that it seeks to base its operations upon definite knowledge drawn from direct observation of the group mind, and upon the application of principles which have been demonstrated to be consistent and relatively constant. There are invisible rulers who control the destinies of millions. It is not generally realized to what extent the words and actions of our most influential public men are dictated by shrewd persons operating behind the scenes.