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, 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 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, it is 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, it is not 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 insensibly decays.  The imagination is extreme quick and agile; but the passions are slow and restive:  For which reason, when any object is presented, that affords a variety of views to the one, and emotions to the other; though the fancy may change its views with great celerity; each stroke will not produce a clear and distinct note of passion, but the one passion will always be mixt and confounded with the other.  According as the probability inclines to good or evil, the passion of joy or sorrow predominates in the composition:  Because the nature of probability is to cast a superior number of views or chances on one side; or, which is the same thing, a superior number of returns of one passion; or since the dispersed passions are collected into one, a superior degree of that passion.  That is, in other words, the grief and joy being intermingled with each other, by means of the contrary views of the imagination, produce by their union the passions of hope and fear.