The "Observations on Man" (1749) is the first systematic effort to

explain the phenomena of mind by the Law of Association
The "Observations on Man" (1749) is the first systematic effort to
explain the phenomena of mind by the Law of Association. It contains
also a philosophical hypothesis, that mental states are produced by the
_vibration_ of infinitesimal particles of the nerves. This analogy,
borrowed from the undulations of the hypothetical substance aether, has
been censured as crude, and has been entirely superseded. But, although
an imperfect analogy, it nevertheless kept constantly before the mind
of Hartley the double aspect of all mental phenomena, thus preventing
erroneous explanations, and often suggesting correct ones. In this
respect, Aristotle and Hobbes are the only persons that can be named as
equally fortunate.