"The Descartian notion of a mechanical body presided over by an independent entity called the soul is replaced, as the "matter" of theoretical physics becomes more attenuated, by the notion of the transformation within the organism of mind-states into body-states, and vice-versa. The dualism of the dead mechanical body, belonging to the world of matter, and the vital transcendental soul, belonging to the spiritual realm, disappears before the increasing insight, derived from physiology on one hand and the investigation of neuroses on the other, of a dynamic interpenetration and conversion within the boundaries of organic structures and functions. Now the physical and the psychal become different aspects of the organic process, in much the same way that heat and light are both aspects of energy, differentiated only by the situation to which they refer and by the particular set of receptors upon which they act."