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 in one moment is determined to consider an object as existent, and in 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 oftener turned to one side than the other, it is 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 objects in their opposite causes, finds such a contrariety as utterly destroys all certainty or established opinion.
Suppose, then, that the object, concerning which we are doubtful, produces either desire or aversion; it is evident, that, according as the mind turns itself to 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 think of those causes, which produce it; and for the same reason, excites grief or uneasiness, from the opposite consideration. So that, as the understanding,in
