Hartley attempted to show that so-called voluntary behavior developed from involuntary, or reflexive, behavior. He used the law of association to explain how involuntary behavior gradually becomes voluntary and then becomes almost involuntary (automatic) again. Involuntary behavior occurs reflexively in response to sensory stimulation. Voluntary behavior occurs in response to one’s ideas or to stimuli not originally associated with the behavior, and vol- untary behavior itself can become so habitual that it too becomes automatic, not unlike involuntary behavior. The basic assumption in Hartley’s expla- nation is that all behavior is at first involuntary and gradually becomes voluntary through the process of association. In the following example, we can see nerves.
These vibrations were called impressions. The impressions reach the brain and cause vibrations in the “infinitesimal, medullary particles,” which cause sensations. Newton had also observed that vibrations in the brain show a certain inertia; that is, they con- tinue vibrating after the impressions causing them cease. This, according to Newton, was why we see a whirling piece of coal as a circle of light. For Hartley, it was the lingering vibrations in the brain follow- ing a sensation that constituted ideas. Ideas, then, were faint replications of sensations. Hartley’s goal was to synthesize Newton’s conception of nerve transmission by vibration with previous versions of empiricism, especially Locke’s. This union of the most pressing questions of philosophy and the most contemporary ideas of physiology would become a hallmark of psychology.