Logical epistemology unduly sways theories of thinking that formulate problems of nonmonotonic reasoning as issues of nondeductive operations on logically phrased beliefs, because the fundamental concepts underlying such reasoning have little to do with logic or belief. These formulations make the resulting theories inappropriately special and hide the characteristic structures of nonmonotonic reasoning amid many unrelated structures. We present a more direct mathematical development of nonmonotonic reasoning free of extraneous logical and epistemological assumptions, and argue that the insights gained in this way exemplify the benefits obtained by approaching psychology as a subject for mathematical investigation through the discipline of rational psychology. For Joseph A. Schatz, teacher and friend 1. Reasoning, logic, and psychology Nonmonotonic reasoning, the study of making and revising assumptions in a reasoned or principled way, needs little introduction in artificial...