  Descartes' work on visual perception is but one instance  of his adopting a naturalistic stance toward conscious mental experience in seeking to explain aspects of such experience. The Passions constitute another. It is sometimes said that Descartes' dualism placed the mind outside nature by rendering it as an immaterial substance. That  is a retrospective judgment from a perspective in which immaterial substances are  automatically deemed “unnatural.” For Descartes and his followers, mind–body interaction  and its laws were included within the domain of natural philosophy or physics  (in the general meaning of the latter term, as the theory of nature). Descartes spoke of regular relations between brain states and the resulting sensory experiences, which his followers, such as Regis, subsequently deemed “laws”  of mind–body relation (see Hatfield 2000). In this way,  Descartes and his followers posited the existence of psychophysical or psychophysiological laws, long before Gustav Fechner (1801–87)  formulated a science of psychophysics in the nineteenth century.