But as their motion is seldom direct, and naturally turns a little to the one side or to the other; for this reason the animal spirits, falling into the contiguous traces, present other related ideas, in lieu of that which the mind desired at first to survey.

This change we are not always sensible of; but continuing still the same train of thought, make use of the related idea which is presented to us and employ it in our reasonings, as if it were the same with what we demanded.

This is the cause of many mistakes and sophisms in philosophy; as will naturally be imagined, and as it would be easy to show, if there was occasion."-- (I.p.

88.) Perhaps it is as well for Hume's fame that the occasion for further physiological speculations of this sort did not arise.

But, while admitting the crudity of his notions and the strangeness of the language in which they are couched, it must in justice be remembered, that what are now known as the elements of the physiology of the nervous system were hardly dreamed of in the first half of the eighteenth century; and, as a further set off to Hume's credit, it must be noted that he grasped the fundamental truth, that the key to the comprehension of mental operations lies in the study of the molecular changes of the nervous apparatus by which they are originated.
Surely no one who is cognisant of the facts of the case, nowadays, doubts that the roots of psychology lie in the physiology of the nervous system.