Hartley’s Explanation of Association
Hartley believed that sense impressions produced vibrations in the nerves, which traveled to the brain and caused similar vibrations in the “med- ullary substance” of the brain. The brain vibrations caused by sense impressions give rise to sensations. After sense impressions cease, there remain in the brain diminutive vibrations that Hartley called vibratiuncles. It is the vibratiuncles that correspond to ideas. Ideas, then, are weaker copies of sensations. Vibratiuncles are like the brain vibrations associ- ated with sensations in every way except they (the vibratiuncles) are weaker. So much for how sense impressions cause ideas; now the question is, how do ideas become associated?
Any Sensations A, B, C, [etc.] by being associated with one another a sufficient Number of Times, get such a Power over the corresponding Ideas a, b, c, [etc.] that any one of the Sensations A, when impressed alone, shall be able to excite in the Mind, b, c, [etc.] the Ideas of the rest. (Hartley, 1749/1834)
Hartley’s notion that experiences consistently occurring together are recorded in the brain as an interrelated package and that experiencing one that Hartley’s (1749/1834) account of the devel- opment of voluntary behavior comes very close to what was later called a conditioned reflex:
