The Cambridge History of English and American Literature in 18 Volumes (1907–21).
Volume X. The Age of Johnson.
§ 17. David Hartley
As we look back upon the development of philosophical problems, it might seem that, for a philosophical writer after Hume, there was but one thing worth doing—to answer him, if possible; and, if that were not possible, to keep silent. But the issue was not quite so clear to his contemporaries. Indeed, his own example did not press it home. It showed, on the contrary, that work of importance might be done in certain departments even when the contradiction was ignored to which Hume had reduced the theory of knowledge. Soon after the publication of A Treatise of Human Nature, valuable writings appeared on psychology, and on moral and political theory; there were also critics of Hume in considerable number; and one of that number had both the insight to trace Hume’s scepticism to its logical origin and the intellectual capacity to set forth a theory of knowledge in which the same difficulty should not arise.
Among the psychologists, the most important place belongs to David Hartley, a physician, and sometime fellow of Jesus college, Cambridge, whose Observations on Man: his frame, his duty, and his expectations appeared in 1749. The rapid march of philosophical thought in the previous forty years was ignored by, and probably unknown to, the author. The whole second part of his book in which he works out a theological theory may be regarded as antiquated. He does not mention Berkeley; he seems never to have heard of David Hume. But the first or psychological part of the book has two striking features: it is a systematic attempt at a physiological psychology, and it developed the theory of the association of ideas in a way which influenced, far more than Hume did, the views of the later associational school of James Mill and his successors. The physiological doctrine was suggested by certain passages in Newton’s Optics. Hartley supposes that the contact of an external object with the sensory nerves excites “vibrations in the æther residing in the pores of these nerves”; these vibrations enter the brain, are “propagated freely every way over the whole medullary substance,” and sensations are the result; further, they leave vestiges or traces behind them, and this is the origin of ideas which depend on minute vibrations or “vibratiuncles.” Motor activity is explained in a similar way. This physiological view is the basis of his whole doctrine of mind, and, more particularly, of the doctrine of association. In respect of the latter doctrine, Hartley wrote under the influence of Locke; but he has left it on record that the suggestion to make use of association as a general principle of psychological explanation came from John Gay, who had written A Dissertation prefixed to Law’s English translation of archbishop King’s Origin of Evil (1731), in which the doctrine was used to explain the connection of morality with private happiness. Hartley offered a physiological explanation of association itself, gave a generalised statement of its laws and applied it to the details of mental life. He did not see, as Hume had seen, the special difficulty of applying it so as to explain judgment, assent, or belief.