T 2.2.8.20

In a word, no ideas can affect each other, either by comparison, or by the passions they separately produce, unless they be united together by some relation, which may cause an easy transition of the ideas, and consequently of the emotions or impressions, attending the ideas; and may preserve the one impression in the passage of the imagination to the object of the other. This principle is very remarkable, because it is analogous to what we have observ'd both concerning the understanding and the passions. Suppose two objects to be presented to me, which are not connected by any kind of relation. Suppose that each of these objects separately produces a passion; and that these two passions are in themselves contrary: We find from experience, that the want of relation in the objects or ideas hinders the natural contrariety of the passions, and that the break in the transition of the thought removes the affections from each other, and prevents their opposition. 'Tis the same case with comparison; and from both these phænomena we may safely conclude, that the relation of ideas must forward the transition of impressions; since its absence alone is able to prevent it, and to separate what naturally shou'd have operated upon each other. When the absence of an object or quality removes any usual or natural effect, we may certainly conclude that its presence contributes to the production of the effect.