 Following the discussion of “originally automatic” motion is this theorem: “If any sensation A, idea B, or motion C, be associated … with any other sensation D, idea E, or muscular motion F, it will, at last, excite d, the simple idea belonging to sensation D, the very idea E, or the very muscular motion F” (OM 1, prop. 20). Through the physical association or neural impulses in the brain, any sensation, idea, or muscular motion can become the stimulus that excites any other idea or muscular motion. One’s heart beats faster at the sight, sound, or thought of something one has learned to fear.