It is 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 wili 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 flxed 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 it is 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, it is 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.