A theme of Kant’s thought, dominant in the pre-critical period, integral to the critical period, and to which he returns in old age, is that humans are part of nature. He was too familiar with the philosophy of Descartes and its problems to subscribe to a mind-body dualism of distinct substances, the one thinking, the other extended. Instead he followed Bilfinger’s heuristic, and already in the Living Forces he had solved Descartes’s puzzle of mind-body interaction by arguing that minds are spatially located within their bodies, and that both are energetic structures that influence each other (1:20–21). In the later Prize Essay (1764), he would judge the Christian notion of immaterial souls as indemonstrable (2:293), an argument he re-echoes in his critical writings as well (A339/B397–A405/B432). Although minds are not necessarily matter in a literal sense, he would argue there that they are probably some kind of energy-bundles commensurate with the material framework of nature. Here in the Universal Natural History, he describes humans as material beings; the makeup of rationality is linked to the constitution of matter as a product of dynamic force (1:335).