T 2.2.4.7

The idea of ourselves is always intimately present to us, and conveys a sensible degree of vivacity to the idea of any other object, to which we are related. This lively idea changes by degrees into a real impression; these two kinds of perception being in a great measure the same, and differing only in their degrees of force and vivacity. But this change must be produc'd with the greater ease, that our natural temper gives us a propensity to the same impression, which we observe in others, and makes it arise upon any slight occasion. In that case resemblance converts the idea into an impression, not only by means of the relation, and by transfusing the original vivacity into the related idea; but also by presenting such materials as take fire from the least spark. And as in both cases a love or affection arises from the resemblance, we may learn that a sympathy with others is agreeable only by giving an emotion to the spirits, since an easy sympathy and correspondent emotions are alone common to relation, acquaintance, and resemblance.