The comprehension and precision with which the law of association is here unfolded as the key to every movement of the mind, and as regulating every wandering thought, cannot be too much admired; it is enough to say that Hartley, who certainly understood more of the power of association than any other man, has added nothing to this short passage, as far as relates to the succession of ideas. He has indeed extended its application in unravelling the fine web of our affections and feelings, by showing how one idea transfers the feeling of pleasure or pain to others associated with it, which is not here noticed. Whether this principle really has all the extent and efficacy ascribed to it by either of these writers will be made the subject of a future inquiry. How well our author understood the question, and how much it had assumed a consistent and systematic form in his mind will appear from the instances he brings in illustration of this intricate and at the time almost unthought-of subject. ‘The train of thoughts or mental discourse is of two sorts. The first is unguided, without design and inconstant; wherein there is no passionate thought to govern and direct those that follow to itself as the end and scope of some desire or other passion; in which case the thoughts are said to wander and seem impertinent one to another as in a dream. Such are commonly the thoughts of men, that are not only without company, but also without care of any thing: though even then their thoughts are as busy as at other times, but without harmony, as the sound which a lute out of tune would yield to any man, or in tune to one that could not play. And yet in this wild ranging of the mind, a man may ofttimes perceive the way of it, and the dependence of one thought upon another. For in a discourse of our present civil war, what could seem more impertinent than to ask (as one did) what was the value of a Roman penny? Yet the coherence to me was manifest enough. For the thoughts of the war introduced the thought of the delivering up the king to his enemies; the thought of that brought in the thought of the delivering up of Christ; and that again the thought of the thirty pence, which was the price of that treason: and thence easily followed that malicious question; and all this in a moment of time; for thought is quick. ‘The second’ [that is the second sort of association] ‘is more constant, as being regulated by some desire, and design. For the impression made by such things as we desire or fear, is strong and permanent, or, if it cease for a time, of quick return; so strong it is sometimes as to hinder and break our sleep. From desire ariseth the thought of some means we have seen produce the like of what we aim at: and from the thought of that, the thought of means to that mean, and so continually till we come to some beginning within our own power.’