Beside good and evil, or, in other words, pain and pleasure, the direct passions frequently arise from a natural impulse or instinct, which is perfectly unaccountable. Of this kind is the desire of punishment to our enemies, and of happiness to our friends; hunger, lust, and a few other bodily appetites. These passions, properly speaking, produce good and evil, and proceed not from them, like the other affections. None of the direct affections seem to merit our particular attention, except hope and fear, which we shall here endeavour to account for. 'Tis evident that the very same event, which, by its certainty, would 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 advanced 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 allowed to fix on either side, but is incessantly tost from one to another, and at one moment is determined 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 though perhaps it may be oftener turned 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 established opinion. Suppose, then, that the object, concerning 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.