The reflex arc concept in psychology, Dewey says, recognizes that the sensory-motor arc is to be taken as the unit of nerve structure, and the type of nerve function. But psychologists do not avail themselves of the full value of this conception, because they still retain in connection with it certain distinctions which were used in the older psychology. "The older dualism between sensation and idea is repeated in the current dualism of peripheral and central structures and functions; the older dualism of body and soul finds a distinct echo in the current dualism of stimulus and response."[117] These rigid distinctions must be set aside, and the separated elements must be viewed as elements in one sensory-motor coordination. Each is to be defined, not as something existing by itself, but as an element functioning in a concrete whole of activity. Thus, if we are to study vision, we must first take vision as a sensory-motor coordination, the act of seeing, and within the whole we may then be able to distinguish certain elements, sensations, or movements, and define them according to their function in the total act of seeing. The reflex arc idea, as commonly employed, takes sensation as stimulus, and movement as response, as if they were actually separate existences, apart from a coordination. Response is said to follow sensation, but it is forgotten that the sensation which preceded was correlated with a response, and that the response which follows is also correlated with sensation. Sound, for instance, is not a mere sensation in itself, apart from sensory-motor coordination. Hearing is an act, and while sound may, for purposes of study, be abstracted from the total, it is not, in itself, independent of the total act of hearing. "But, in spite of all this, it will be urged, there is a distinction between stimulus and response, between sensation and motion. Precisely; but we ought now to be in a condition to ask of what nature is the distinction, instead of taking it for granted as a distinction somehow lying in the existence of the facts themselves."[118] The distinction which is to be made between them must be made on a teleological basis. "The fact is that stimulus and response are not distinctions of existence, but teleological distinctions, that is, distinctions of function, or part played, with reference to reaching or maintaining an end."[119] There are two kinds of teleological distinction that can be made between stimulus and response, or rather, the teleological interpretation has two phases.