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. Evils that are _certain_ have sometimes the same effect in producing fear, as the possible or impossible. Thus, a man in a strong prison well-guarded, without the least means of escape, trembles at the thought of the rack, to which he is sentenced. This happens only when the certain evil is terrible and confounding; in which case the mind continually rejects it with horror, while it continually presses in upon the thought. The evil is there fixed and established, but the mind cannot endure to fix upon it; from which fluctuation and uncertainty there arises a passion of much the same appearance with fear. But 'tis not only where good or evil is uncertain, as to its _existence_, but also as to its _kind_, that fear or hope arises. Let one be told by a person, whose veracity he cannot doubt of, that one of his sons is suddenly killed, 'tis evident the passion this event would occasion, would not settle into pure grief, till he got certain information which of his sons he had lost. Here there is an evil certain, but the kind of it uncertain: consequently the fear we feel on this occasion is without the least mixture of joy, and arises merely from the fluctuation of the fancy betwixt its objects. And though each side of the question produces here the same passion, yet that passion cannot settle, but receives from the imagination a tremulous and unsteady motion, resembling in its cause, as well as in its sensation, the mixture and contention of grief and joy.