The passions of fear and hope may arise when the chances are equal on both sides, and no superiority can be discovered 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 tossed 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. Increase 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 increased 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 increase that part of the composition by the increase 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 coloured ray of the sun passing through a prism, is a composition of two others, when, as you diminish or increase 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. Probability is of two kinds, either when the object is really in itself uncertain, and to be determined by chance; or when, though the object be already certain, yet 'tis uncertain to our judgment, which finds a number of proofs on each side of the question. Both these kinds of probabilities cause fear and hope; which can only proceed from that property, in which they agree, viz. the uncertainty and fluctuation they bestow on the imagination by the contrariety of views which is common to both. 'Tis a probable good or evil that commonly produces hope or fear; because probability, being a wavering and unconstant method of surveying an object, causes naturally a like mixture and uncertainty of passion. But we may observe, that wherever, from other causes, this mixture can be produced, the passions of fear and hope will arise, even though there be no probability; which must be allowed to be a convincing proof of the present hypothesis. We find that an evil, barely conceived as _possible_, does sometimes produce fear; especially if the evil be very great. A man cannot think of excessive pains and tortures without trembling, if he be in the least danger of suffering them. The smallness of the probability is compensated by the greatness of the evil; and the sensation is equally lively, as if the evil were more probable. One view or glimpse of the former has the same effect as several of the latter.