T 2.3.9.19

The passions of fear and hope may arise when the chances are equal on both sides, and no superiority can be discover'd in the one above the other. Nay, in this situation the passions are rather the strongest, as the mind has then the least foundation to rest upon, and is toss'd with the greatest uncertainty. Throw in a superior degree of probability to the side of grief, you immediately see that passion diffuse itself over the composition, and tincture it into fear. Encrease the probability, and by that means the grief, the fear prevails still more and more, till at last it runs insensibly, as the joy continually diminishes, into pure grief. After you have brought it to this situation, diminish the grief, after the same manner that you encreas'd it; by diminishing the probability |on that side, and you'll see the passion clear every moment, 'till it changes insensibly into hope; which again runs, after the same manner, by slow degrees, into joy, as you encrease that part of the composition by the encrease of the probability. Are not these as plain proofs, that the passions of fear and hope are mixtures of grief and joy, as in optics 'tis a proof, that a colour'd ray of the sun passing thro' a prism, is a composition of two others, when, as you diminish or encrease the quantity of either, you find it prevail proportionably more or less in the composition? I am sure neither natural nor moral philosophy admits of stronger proofs.