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.
From these principles we may account for a phaenomenon in the passions, which at first sight seems very extraordinary, viz, that surprize is apt to change into fear, and every thing that is unexpected affrights us.

The most obvious conclusion from this is, that human nature is in general pusillanimous; since upon the sudden appearance of any object.
we immediately conclude it to be an evil, and without waiting till we can examine its nature, whether it be good or bad, are at first affected with fear.