When the contrary passions arise from objects entirely different, they take place alternately, the want of relation in the ideas separating the impressions from each other, and preventing their opposition. Thus, when a man is afflicted for the loss of a lawsuit, and joyful for the birth of a son, the mind running from the agreeable to the calamitous object, with whatever celerity it may perform this motion, can scarcely temper the one affection with the other, and remain betwixt them in a state of indifference.
It more easily attains that calm situation, when the same event is of a mixt nature, and contains something adverse and something prosperous in its different circumstances. For in that case, both the passions, mingling with each other by means of the relation, become mutually destructive, and leave the mind in perfect tranquillity.
But suppose, in the third place, that the object is not a compound of good or evil, but is considered as probable or improbable in any degree; in that case I assert, that the contrary passions will both of them be present at once in the soul, and, instead of destroying and tempering each other, will subsist together, and produce a third impression or affection by their union. Contrary passions are not capable of destroying each other, except when their contrary movements exactly rencounter, and are opposite in their direction, as well as in the sensation they produce. This exact rencounter depends upon the relations of those ideas from which they are derived, and is more or less perfect, according to the degrees of the relation. In the case of probability, the contrary chances are so far related that they determine concerning the existence or non-existence of the same object. But this relation is far from being perfect, since some of the chances lie on the side of existence, and others on that of non-existence, which are objects altogether incompatible. 'Tis impossible, by one steady view, to survey the opposite chances, and the events dependent on them; but 'tis necessary that the imagination should run alternately from the one to the other. Each view of the imagination produces its peculiar passion, which decays away by degrees, and is followed by a sensible vibration after the stroke. The incompatibility of the views keeps the passions from shocking in a direct line, if that expression may be allowed; and yet their relation is sufficient to mingle their fainter emotions. 'Tis after this manner that hope and fear arise from the different mixture of these opposite passions of grief and joy, and from their imperfect union and conjunction.
Upon the whole, contrary passions succeed each other alternately, when they arise from different objects; they mutually destroy each other, when they proceed from different parts of the same; and they subsist, both of them, and mingle together, when they are derived from the contrary and incompatible chances or possibilities on which any one object depends. The influence of the relations of ideas is plainly seen in this whole affair. If the objects of the contrary passions be totally different, the passions are like two opposite liquors in different bottles, which have no influence on each other. If the objects be intimately connected, the passions are like an alkali and an acid, which, being mingled, destroy each other. If the relation be more imperfect, and consists in the contradictory views of the same object, the passions are like oil and vinegar, which, however mingled, never perfectly unite and incorporate.
As the hypothesis concerning hope and fear carries its own evidence along with it, we shall be the more concise in our proofs. A few strong arguments are better than many weak ones.
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.
But they are not only possible evils that cause fear, but even some allowed to be impossible; as when we tremble on the brink of a precipice, though we know ourselves to be in perfect security, and have it in our choice whether we will advance a step farther. This proceeds from the immediate presence of the evil, which influences the imagination in the same manner as the certainty of it would do; but being encountered by the reflection on our security, is immediately retracted, and causes the same kind of passion, as when, from a contrariety of chances, contrary passions are produced.