Whatever new emotion, therefore, proceeds from the former naturally encreases the courage; as the same emotion, proceeding from the latter, augments the fear; by the relation of ideas, and the conversion of the inferior emotion into the predominant.

Hence it is that in martial discipline, the uniformity and lustre of our habit, the regularity of our figures and motions, with all the pomp and majesty of war, encourage ourselves and allies; while the same objects in the enemy strike terror into us, though agreeable and beautiful in themselves.
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 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 encreases 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 encrease them, by producing an opposition in our motives and principles.
The same effect follows whether the opposition arises from internal motives or external obstacles.