Naturaleza humana
Casmann is important to the history of anthropology and psychology. He began the separation of these two subjects from the Aristotelian framework of metaphysics, becoming a classical representative of the secularization of science in the early modern period. During his time at Steinfurt he produced the work "Psychologia Anthropologica, sive doctrina animae Humanae" (Hanau, 1594). There he consolidated the use of the term "anthropology" coined by Magnus Hundt (1449-1519). During his time in Stade, he wrote the second volume of "Psychologia Anthropologica" (Hanau, 1596) in which he described the construction of the human body. In 1594, Casmann defined anthropology as "the doctrine of human nature. Human nature is an essence partaking of two worlds, the spiritual and the corporeal, yet united in one vehicle." This definition is still considered valid today.