Objective thought, as we have seen, posited a world of objects in which different objects as well as different parts of the same object were related in a purely external manner. Mechanistic physiology incorporated the living body into this causal system by converting human behaviour into a pattern of stimulus-response. Thus the stimulus was thought to impinge on a particular sense organ which in turn transmitted sensations to the brain and thereby produced a predictable perception. However, neural physiology found itself forced to abandon this purely mechanistic approach when it became evident that damaging centres or conductors resulted in subjects' loss of discrimination — frequently progressive — among stimuli rather than in an outright loss of "certain qualities of sensation or of certain sensory givens". What was at stake in such injuries was the organization of the sensory fields, the modification or collapse of figure-background structures. Consequently, modern physiology itself replaced the mechanistic stimulus-response model with the notion of an organism which meets and relates to stimulation in a variety of ways. Since the stimulation of a sense organ did not in and of itself invariably produce a perception, it became necessary to speak of an "attuning", or disposition, of the organism to the excitation and to regard perception as a "psychophysical" rather than a purely physical and physiological event.