We shall here enumerate some of those circumstances, which render a passion calm or violent, which heighten or diminish any emotion.

It is a property in human nature, that any emotion, which attends a passion, is easily converted into it; though in their natures they be originally different from, and even contrary to each other. It is true, in order to cause a perfect union amongst passions, and make one produce the other, there is always required a double relation, according to the theory above delivered. But when two passions are already produced by their separate causes, and are both present in the mind, they readily mingle and unite; though they have but one relation, and sometimes without any. The predominant passion swallows up the inferior, and converts it into itself. The spirits, when once excited, easily receive a change in their direction; and it is natural to imagine, that this change will come from the prevailing affection. The connexion is in many cases closer between any two passions, than between any passion and indifference.

When a person is once heartily in love, the little faults and caprices of his mistress, the jealousies and quarrels, to which that commerce is so subject; however unpleasant they be, and rather connected with anger and hatred; are yet found, in many instances, to give additional force to the prevailing passion. It is a common artifice of politicians, when they would affect any person very much by a matter of fact, of which they intend to inform him, first to excite his curiosity; delay as long as possible the satisfying of it; and by that means raise his anxiety and impatience to the utmost, before they give him a full insight into the business. They know, that this curiosity will precipitate him into the passion, which they purpose to raise, and will assist the object in its influence on the mind. A soldier advancing to battle, is naturally inspired with courage and confidence, when he thinks on his friends and fellow-soldiers; and is struck with fear and terror, when he reflects on the enemy. Whatever new emotion therefore proceeds from the former, naturally encreases the courage; as the same emotion proceeding from the latter, augments the fear. Hence in martial discipline, the uniformity and lustre of habit, the regularity of figures and motions, with all the pomp and majesty of war, encourage ourselves and our allies; while the same objects in the enemy strike terror into us, though agreeable and beautiful in themselves.

Hope is, in itself, an agreeable passion, and allied to friendship and benevolence; yet is it able sometimes to blow up anger, when that is the predominant passion. Spes addita suscitat iras. Virg.

Since passions, however independent, are naturally transfused into each other, if they be both present at the same time; it follows, that when good or evil is placed in such a situation as to cause any particular emotion, besides its direct passion of desire or aversion, this latter passion must acquire new force and violence.

This often happens, when any object excites contrary passions. For it is observable, that an opposition of passions commonly causes a new emotion in the spirits, and produces more disorder than the concurrence of any two affections of equal force. This new emotion is easily converted into the predominant passion, and in many instances, is observed to encrease its violence, beyond the pitch, at which it would have arrived, had it met with no opposition. Hence we naturally desire what is forbid, and often take a pleasure in performing actions, merely because they are unlawful. The notion of duty, when opposite to the passions, is not always able to overcome them; and when it fails of that effect, is apt rather to encrease and irritate them, by producing an opposition in our motives and principles.

The same effect follows, whether the opposition arise from internal motives or external obstacles. The passion commonly acquires new force in both cases. The efforts, which the mind makes to surmount the obstacle, excite the spirits, and enliven the passion.

Uncertainty has the same effect as opposition. The agitation of the thought, the quick turns which it makes from one view to another, the variety of passions which succeed each other, according to the different views: All these produce an emotion in the mind; and this emotion transfuses itself into the predominant passion.

Security, on the contrary, diminishes the passions. The mind, when left to itself, immediately languishes; and in order to preserve its ardour, must be every moment supported by a new flow of passion. For the same reason, despair, though contrary to security, has a like influence.

Nothing more powerfully excites any affection than to conceal some part of its object, by throwing it into a kind of shade, which, at the same time that it shows enough to prepossess us in favour of the object, leaves still some work for the imagination. Besides that obscurity is always attended with a kind of uncertainty; the effort, which the fancy makes to compleat the idea, rouzes the spirits, and gives an additional force to the passion.

As despair and security, though contrary, produce the same effects; so absence is observed to have contrary effects, and in different circumstances, |either encreases or diminishes our affection. Rochefoucault has very well remarked, that absence destroys weak passions, but encreases strong; as the wind extinguishes a candle, but blows up a fire. Long absence naturally weakens our idea, and diminishes the passion: But where the affection is so strong and lively as to support itself, the uneasiness, arising from absence, encreases the passion, and gives it new force and influence.

When the soul applies itself to the performance of any action, or the conception of any object, to which it is not accustomed, there is a certain unpliableness in the faculties, and a difficulty of the spirits moving in their new direction. As this difficulty excites the spirits, it is the source of wonder, surprize, and of all the emotions, which arise from novelty; and is, in itself, agreeable, like every thing which enlivens the mind to a moderate degree. But though surprize be agreeable in itself, yet, as it puts the spirits in agitation, it not only augments our agreeable affections, but also our painful, according to the foregoing principle. Hence every thing that is new, is most affecting, and gives us either more pleasure or pain, than what, strictly speaking, should naturally follow from it. When it often returns upon us, the novelty wears off; the passions subside; the hurry of the spirits is over; and we survey the object with greater tranquillity.

The imagination and affections have a close union together. The vivacity of the former gives force to the latter. Hence the prospect of any pleasure, with which we are acquainted, affects us more than any other pleasure, which we may own superior, but of whose nature we are wholly ignorant. Of the one we can form a particular and determinate idea: The other we conceive under the general notion of pleasure.

Any satisfaction, which we lately enjoyed, and of which the memory is fresh and recent, operates on the will with more violence, than another of which the traces are decayed and almost obliterated.

A pleasure, which is suitable to the way of life, in which we are engaged, excites more our desire and appetite than another, which is foreign to it.

Nothing is more capable of infusing any passion into the mind, than eloquence, by which objects are represented in the strongest and most lively colours. The bare opinion of another, especially when enforced with passion, will cause an idea to have an influence upon us, though that idea might otherwise have been entirely neglected.

It is remarkable, that lively passions commonly attend a lively imagination. In this respect, as well as in others, the force of the passion depends as much on the temper of the person, as on the nature and situation of the object.

What is distant, either in place or time, has not equal influence with what is near and contiguous.

I pretend not to have here exhausted this subject. It is sufficient for my purpose, if I have made it appear, that, in the production and conduct of the passions, there is a certain regular mechanism, which is susceptible of as accurate a disquisition, as the laws of motion, optics, hydrostatics, or any part of natural philosophy.