T 2.3.9.16

But suppose, in the third place, that the object is not a compound of good or evil, but is consider'd as probable or improbable in any degree; in that case I assert, that the contrary passions will both of them be present at once in the soul, and instead of destroying and tempering each other, will subsist together, and produce a third impression or affection by their union. Contrary passions are not capable of destroying each other, except when their contrary movements exactly rencounter, and are opposite in their direction, as well as in the sensation they produce. This exact rencounter depends upon the relations of those ideas, from which they are deriv'd, and is more or less perfect, according to the degrees of the relation. In the case of probability the contrary chances are so far related, that they determine concerning the existence or non-existence of the same object. But this relation is far from being perfect; since some of the chances lie on the side of existence, and others on that of non-existence; which are objects altogether incompatible. 'Tis impossible by one steady view to survey the opposite chances, and the events dependent on them; but 'tis necessary, that the imagination shou'd run alternately from the one to the other. Each view of the imagination produces its peculiar passion, which decays away by degrees, and is follow'd by a sensible vibration after the stroke. The incompatibility of the views keeps the passions from shocking in a direct line, if that expression may be allow'd; and yet their relation is sufficient to mingle their fainter emotions. |'Tis after this manner that hope and fear arise from the different mixture of these opposite passions of grief and joy, and from their imperfect union and conjunction.