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.