For Descartes there are but two agents in the universe -- matter and mind. There is no possibility of interaction between them.All changes in bodies have to be explained mechanically.But since it is a spiritual being retaining spiritual activities, intrinsically independent of the body, it is, as St.Thomas says, non totaliter immersa , not entirely submerged in matter, as are the actuating forms of the animal and the plant.With him genuine scientific and philosophic treatment of the subject begins, and the position to which he advanced it is among the finest evidences of both his encyclopedic knowledge and his metaphysical genius.His chief discussions of the topic are to be found in his peri psyches and peri zoön geneseos .The human soul was conceived as a spiritual substantial principle containing virtually the lower faculties of sensory and vegetative life.It is through this lower organic capacity that it is enabled to inform and animate the matter of the body.
With respect to the question of the origin of life Aristotle, followed by Albertus Magnus, St.They are called substantial principles because combined they result in a being; but they are incomplete beings in themselves, incapable of existing alone.To the form is due the specific nature of the being with its activities and properties. (See FORM; MATTER.) For Aristotle, in the case of living natural bodies the vital principle, psyche is the form.The human soul, however, endowed with rationality is of a higher grade.It is form of the body which it animates, not in virtue of its rationality but through the vegetative and sentient faculties which it also possesses.