event, which by its certainty wou'd produce grief or joy, gives always rise to fear or hope, when only probable and uncertain. In order, therefore, to understand the reason why this circumstance makes such a considerable difference, we must reflect on what I have already advanc'd in the preceding book concerning the nature of probability.
Probability arises from an opposition of contrary chances or causes, by which the mind is not allow'd to fix on either side, but is incessantly tost from one to another, and at one moment is determin'd to consider an object as existent, and at another moment as the contrary. The imagination or understanding, call it which you please, fluctuates betwixt the opposite views; and tho' perhaps it may be oftner turn'd to the one side than the other, 'tis impossible for it, by reason of the opposition of causes or chances, to rest on either. The pro and con of the question alternately prevail; and the mind, surveying the object in its opposite principles, finds such a contrariety as utterly destroys all certainty and establish'd opinion.
Suppose, then, that the object, conceding whose reality we are doubtful, is an object either of desire or aversion, 'tis evident, that, according as the mind turns itself either to the one side or the other, it must feel a momentary impression of joy or sorrow. An object, whose existence we desire, gives satisfaction, when we reflect on those causes, which produce it; and for the same reason excites grief or uneasiness from the opposite consideration: So that as the understanding, in all probable questions, is divided betwixt the contrary points of view, the affections must in the same manner be divided betwixt opposite emotions.
Now if we consider the human mind, we shall find, that with regard to the passions, 'tis not of the nature of a wind instrument of music, which in running over all the notes immediately loses the sound after the breath ceases; but rather resembles a string-instrument, where after each stroke the vibrations still retain some sound, which gradually and
