What is a stimulus? A stimulus is a physical record of an adaptation, specifically that of a sensitive system registering the difference between one momentary impression or attack upon it and another that just precedes it. Can the stimulus-world in which our behavior takes shape be seen to have a historical dimension? Perhaps so. The rapid crowding and kaleidoscoping of images and experiences that characterized the early-20th-century metropolis were both a physical and psychoneurological condition. Read article here.