There is not in my opinion any other natural cause, why security diminishes the passions, than because it removes that uncertainty, which encreases them.  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.

It is certain nothing more powerfully animates 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 chews enough to pre-possess 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 to each other, produce the same effects; so absence is observed to have contrary effects, and in different circumstances either encreases or diminishes our affections.  The Duc de La Rochefoucault has very well observed, 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 idea is so strong and lively as to support itself, the uneasiness, arising from absence, encreases the passion and gives it new force and violence.

SECT.  V OF THE EFFECTS OF CUSTOM

But nothing has a greater effect both to encrease and diminish our passions, to convert pleasure into pain, and pain into pleasure, than custom and repetition.  Custom has two original effects upon the mind, in bestowing a facility in the performance of any action or the conception of any object; and afterwards a tendency or inclination towards it; and from these we may account for all its other effects, however extraordinary.

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 spirit’s 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 very agreeable, like every thing, which inlivens 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, that every emotion, which precedes or attends a passion, is easily converted into it.  Hence every thing, that is new, is most affecting, and gives us either more pleasure or pain, than what, strictly speaking, naturally belongs to 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 objects with greater tranquillity.