Thus we have established two truths without any obstacle or difficulty, _that 'tis from natural principles this variety of causes excite pride and humility_, and _that 'tis not by a different principle each different cause is adapted to its passion_. We shall now proceed to inquire how we may reduce these principles to a lesser number, and find among the causes something common on which their influence depends. 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. '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 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 completed. 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; though with this remarkable difference, that ideas are associated by resemblance, contiguity, and causation, and impressions only by resemblance.