A detailed reply to criticisms by D. D. Wickens (J. exp. Psychol., 1938, 22, 101-123) and A. A. Campbell (J. exp. Psychol., 1938, 22, 225-243) of the author's attitudinal or central theory of human conditioning. (PsycINFO Database Record (c) 2006 APA, all rights reserved)