In order to this we must reflect on certain properties of human nature, which though they have a mighty influence on every operation both of the understanding and passions, are not commonly much insisted on by philosophers.  The first of these is the association of ideas, which I have so often observed and explained.  It is impossible for the mind to fix itself steadily upon one idea for any considerable time; nor can it by its utmost efforts ever arrive at such a constancy.  But however changeable our thoughts may be, they are not entirely without rule and method in their changes.  The rule, by which they proceed, is to pass from one object to what is resembling, contiguous to, or produced by it.  When one idea is present to the imagination, any other, united by these relations, naturally follows it, and enters with more facility by means of that introduction.

The second property I shall observe in the human mind is a like association of impressions.  All resembling impressions are connected together, and no sooner one arises than the rest immediately follow.  Grief and disappointment give rise to anger, anger to envy, envy to malice, and malice to grief again, till the whole circle be compleated.  In like manner our temper, when elevated with joy, naturally throws itself into love, generosity, pity, courage, pride, and the other resembling affections.  It is difficult for the mind, when actuated by any passion, to confine itself to that passion alone, without any change or variation.  Human nature is too inconstant to admit of any such regularity.  Changeableness is essential to it.  And to what can it so naturally change as to affections or emotions, which are suitable to the temper, and agree with that set of passions, which then prevail?  It is evident, then, there is an attraction or association among impressions, as well as among ideas; though with this remarkable difference, that ideas are associated by resemblance, contiguity, and causation; and impressions only by resemblance.

In the third place, it is observable of these two kinds of association, that they very much assist and forward each other, and that the transition is more easily made where they both concur in the same object.  Thus a man, who, by any injury from another, is very much discomposed and ruffled in his temper, is apt to find a hundred subjects of discontent, impatience, fear, and other uneasy passions; especially if he can discover these subjects in or near the person, who was the cause of his first passion.  Those principles, which forward the transition of ideas, here concur with those, which operate on the passions; and both uniting in one action, bestow on the mind a double impulse.  The new passion, therefore, must arise with so much greater violence, and the transition to it must be rendered so much more easy and natural.

Upon this occasion I may cite the authority of an elegant writer, who expresses himself in the following manner.