Are not these as plain proofs, that the passions of fear and hope are mixtures of grief and joy, as in optics it is a proof, that a coloured ray of the sun passing through a prism, is a composition of two others, when, as you diminish or encrease the quantity of either, you find it prevail proportionably more or less in the composition? I am sure neither natural nor moral philosophy admits of stronger proofs.

Probability is of two kinds, either when the object is really in itself uncertain, and to be determined by chance; or when, though the object be already certain, yet it is uncertain to our judgment, which finds a number of proofs on each side of the question. Both these kinds of probabilities cause fear and hope; which can only proceed from that property, in which they agree, viz, the uncertainty and fluctuation they bestow on the imagination by that contrariety of views, which is common to both.

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 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 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.

From these principles we may account for a phænomenon 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 phænomenon 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 relation 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 phænomenon.

Ut assidens implumibus pullis avis
    Serpentium allapsus timet,
Magis relictis; non, ut adsit, auxilî
    Latura plus præsentibus.

[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.]

But this principle of the connexion of fear with uncertainty I carry farther, and observe that any doubt produces that passion, even though it presents nothing to us on any side but what is good and desireable. A virgin, on her bridalnight goes to bed full of fears and apprehensions, though she expects nothing but pleasure of the highest kind, and what she has long wished for. The newness and greatness of the event, the confusion of wishes and joys so embarrass the mind, that it knows not on what passion to fix itself; from whence arises a fluttering or unsettledness of the spirits which being, in some degree, uneasy, very naturally degenerates into fear.

Thus we still find, that whatever causes any fluctuation or mixture of passions, with any degree of uneasiness, always produces fear, or at least a passion so like it, that they are scarcely to be distinguished.

I have here confined myself to the examination of hope and fear in their most simple and natural situation, without considering all the variations they may receive from the mixture of different views and reflections. Terror, consternation, astonishment, anxiety, and other passions of that kind, are nothing but different species and degrees of fear. It is easy to imagine how a different situation of the object, or a different turn of thought, may change even the sensation of a passion; and this may in general account for all the particular sub-divisions of the other affections, as well as of fear. Love may shew itself in the shape of tenderness, friendship, intimacy, esteem, good-will, and in many other appearances; which at the bottom are the same affections; and arise from the same causes, though with a small variation, which it is not necessary to give any particular account of. It is for this reason I have all along confined myself to the principal passion.

The same care of avoiding prolixity is the reason why I wave the examination of the will and direct passions, as they appear in animals; since nothing is more evident, than that they are of the same nature, and excited by the same causes as in human creatures. I leave this to the reader's own observation; desiring him at the same time to consider the additional force this bestows on the present system.

But methinks we have been not a little inattentive to run over so many different parts of the human mind, and examine so many passions, without taking once into the consideration that love of truth, which was the first source of all our enquiries. Twill therefore be proper, before we leave this subject, to bestow a few reflections on that passion, and shew its origin in human nature. It is an affection of so peculiar a kind, that it would have been impossible to have treated of it under any of those heads, which we have examined, without danger of obscurity and confusion.

Truth is of two kinds, consisting either in the discovery of the proportions of ideas, considered as such, or in the conformity of our ideas of objects to their real existence. It is certain, that the former species of truth, is not desired merely as truth, and that it is not the justness of our conclusions, which alone gives the pleasure. For these conclusions are equally just, when we discover the equality of two bodies by a pair of compasses, as when we learn it by a mathematical demonstration; and though in the one case the proofs be demonstrative, and in the other only sensible, yet generally speaking, the mind acquiesces with equal assurance in the one as in the other. And in an arithmetical operation, where both the truth and the assurance are of the same nature, as in the most profound algebraical problem, the pleasure is very inconsiderable, if rather it does not degenerate into pain: Which is an evident proof, that the satisfaction, which we sometimes receive from the discovery of truth, proceeds not from it, merely as such, but only as endowed with certain qualities.

The first and most considerable circumstance requisite to render truth agreeable, is the genius and capacity, which is employed in its invention and discovery. What is easy and obvious is never valued; and even what is in itself difficult, if we come to the knowledge of it without difficulty, and without any stretch of thought or judgment, is but little regarded. We love to trace the demonstrations of mathematicians; but should receive small entertainment from a person, who should barely inform us of the proportions of lines and angles, though we reposed the utmost confidence both in his judgment and veracity. In this case it is sufficient to have ears to learn the truth. We never are obliged to fix our attention or exert our genius; which of all other exercises of the mind is the most pleasant and agreeable.