In this chapter, the author examines critical historical and philosophical problems and issues that shaped psychology's development. Neurological and physiological explanations lead to questions of a biochemical, chemical, or physical nature. At conception, according to psychogenic identity theory, there was one psyche and one body. The late nineteenth-century leadership of Charles Sanders Peirce and William James, pragmatism became a major philosophical movement. The material world is regarded as derivative—an intellectual or philosophical product that has its origin in the world of experience. For the idealist, psychology is the science that studies mental processes and experience. The material world is a construction—a mere by-product of a more important reality. The Aristotelian notion of final causation should not be confused with teleological interpretations of the world encountered in numerous theological beliefs. For Aristotle, knowledge of causation rests on understanding antecedent conditions, material, form, and the purpose for which a thing was intended.