Descartes proposed that the mind exists outside of the physical universe, in a realm that is not purely deterministic, in a realm that can not be explored by science, Much later, when psychology emerged as a discipline distinct from philosophy and physiology, it was to grapple with the question of whether or not to define itself as the study of mind.
Psychologists with a mechanistic bent eventually defined psychology as the study of observable behavior. They assumed that the behavior would follow specific laws of nature, and that it would be controlled by the environment and by physiology with no influence from the mind. Many psychologists were uncomfortable with the mechanistic definition of their discipline, and desired to explore a more expanded perspective of human experience, one that included the mind as a causative factor.