T 2.3.9.12

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 |insensibly decays. The imagination is extremeextremely 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; tho' 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 dispers'd 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.