Since passions, however independent, are naturally transfused into each other, if they are 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 beside its direct passion of desire or aversion, that latter passion must acquire new force and violence. This happens, among other cases, whenever any object excites contrary passions. For 'tis 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 increases its violence beyond the pitch it would have arrived at had it met with no opposition. Hence we naturally desire what is forbid, and take a pleasure in performing actions, merely because they are unlawful. The notion of duty, when opposite to the passions, is seldom able to overcome them; and, when it fails of that effect, is apt rather to increase them, by producing an opposition in our motives and principles. The same effect follows, whether the opposition arises from internal motives or external obstacles. The passion commonly acquires new force and violence in both cases. The efforts which the mind makes to surmount the obstacle, excite the spirits and enliven the passion. Uncertainty has the same influence as opposition. The agitation of the thought, the quick turns it makes from one view to another, the variety of passions which succeed each other, according to the different views; all these produce an agitation in the mind, and transfuse themselves into the predominant passion. There is not, in my opinion, any other natural cause why security diminishes the passions, than because it removes that uncertainty which increases 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. 'Tis certain, nothing more powerful 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 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 complete the idea, rouses the spirits, and gives an additional force to the passion.