In order to this we must reflect on certain properties of human nature, which tho' 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 observ'd and explain'd. 'Tis 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 produc'd 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. 'Tis 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? 'Tis evident, then, there is an attraction or association among impressions, as well as among ideas; tho' with this remarkable difference, that ideas are associated by resemblance, contiguity, and causation; and impressions only by resemblance.
In the third place, 'tis observable of these two kinds of
