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.  This I say is the most obvious conclusion; but upon farther examination we shall find that the phaenomenon is otherwise to be accounted for.  The suddenness and strangeness of an appearance naturally excite a commotion in the mind, like every thing for which we are not prepared, and to which we are not accustomed.  This commotion, again, naturally produces a curiosity or inquisitiveness, which being very violent, from the strong and sudden impulse of the object, becomes uneasy, and resembles in its fluctuation and uncertainty, the sensation of fear or the mixed passions of grief and joy.  This image of fear naturally converts into the thing itself, and gives us a real apprehension of evil, as the mind always forms its judgments more from its present disposition than from the nature of its objects.

Thus all kinds of uncertainty have a strong connexion with fear, even though they do not cause any opposition of passions by the opposite views and considerations they present to us.  A person, who has left his friend in any malady, will feel more anxiety upon his account, than if he were present, though perhaps he is not only incapable of giving him assistance, but likewise of judging of the event of his sickness.  In this case, though the principal object of the passion, viz, the life or death of his friend, be to him equally uncertain when present as when absent; yet there are a thousand little circumstances of his friend’s situation and condition, the knowledge of which fixes the idea, and prevents that fluctuation and uncertainty so near allyed to fear.  Uncertainty is, indeed, in one respect as near allyed to hope as to fear, since it makes an essential part in the composition of the former passion; but the reason, why it inclines not to that side, is, that uncertainty alone is uneasy, and has a reladon of impressions to the uneasy passions.

It is thus our uncertainty concerning any minute circumstance relating to a person encreases our apprehensions of his death or misfortune.  Horace has remarked this phaenomenon.

UT ASSIDENS IMPLUMI bus Pullus Avis
SERPENTIUM ALLAPSUS TIRNET,
MAGIS RELICTIS; non, UT Adsit, AUXILI
LATURA plus PRESENTIBUS.

[As a bird, watching over her fledgelings, is more afraid of their being attacked by snakes if she were to leave them even though, were she to stay, she would not be any more capable of helping them, when they were with her.]