T 2.2.4.10

In order to produce a perfect relation betwixt two objects, 'tis requisite, not only that the imagination be convey'd from one to the other by resemblance, contiguity or causation, but also that it return back from the second to the first with the same ease and facility. At first sight this may seem a necessary and unavoidable consequence. If one object resemble another, the latter object must necessarily resemble the former. If one object be the cause of another, the second object is effect to its cause. 'Tis the same case with contiguity: And therefore the relation being always reciprocal, it may be thought, that the return of the imagination |from the second to the first must also, in every case, be equally natural as its passage from the first to the second. But upon farther examination we shall easily discover our mistake. For supposing the second object, beside its reciprocal relation to the first, to have also a strong relation to a third object; in that case the thought, passing from the first object to the second, returns not back with the same facility, tho' the relation continues the same; but is readily carry'd on to the third object, by means of the new relation, which presents itself, and gives a new impulse to the imagination. This new relation, therefore, weakens the tie betwixt the first and second objects. The fancy is by its very nature wavering and inconstant; and considers always two objects as more strongly related together, where it finds the passage equally easy both in going and returning, than where the transition is easy only in one of these motions. The double motion is a kind of a double tie, and binds the objects together in the closest and most intimate manner.