[Development of the Cartesian "Mechanics of vision" and its limits]

In his "Objections and Replies" to the "Meditations" Descartes developed a concept of sensory perception as a three-stage mechanism. Firstly, it consists of a purely mechanical process which comprises an excitation of the sensory organs leading to motion in the nerves. Secondly, he understood perception as a psychophysical interaction in which a specific signal pattern representing the surroundings is presented to the soul. Finally, in stage three, classification and judgement of the object presented complete the perceptual process. Ernst Cassirer suggests that Descartes' conception of the soul is elucidated more precisely in his physiological writings, in which physiology is considered as a field for the application of philosophical analysis. In the present paper an attempt is made to demonstrate that Descartes' mechanistic concept of vision led to a definition of perception as a active process within the soul. Based on Kepler's theory of the inverted retinal image and a rejection of the traditional idea of light generated within the eye, Descartes arrived at the hypothesis that one must strictly differentiate between the object image and its perceptual correlate. Since the objective physical conditions do not suffice for a complete understanding of perception, a restricted validity of the mechanistic theory of vision is the consequence. Thus Descartes transposed the question of perception as an active process from neurophysiology to psychology and established its relevance within his own metaphysics, hereby laying the foundations for a psychological discussion of theories of vision in British Sensism.