David Hume

“ There is not in philosophy a subject of more nice speculation than this of the different causes and effects of the calm and violent passions. 'Tis evident passions influence not the will in proportion to their violence, or the disorder they occasion in the temper; but on the contrary, that when a passion has once become a settled principle of action, and is the predominant inclination of the soul, it commonly produces no longer any sensible agitation. ”

“ 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 inliven 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. ”

“ As repeated custom and its own force have made every thing yield to it, it directs the actions and conduct without that opposition and emotion, which so naturally attend every momentary gust of passion. We must, therefore, distinguish betwixt a calm and a weak passion; betwixt a violent and a strong one. But notwithstanding this, 'tis certain, that when we wou'd govern a man, and push him to any action, 'twill commonly be better policy to work upon the violent than the calm passions, and rather take him by his inclination, than what is vulgarly call'd his reason. ”

“ Since passions, however independent, are naturally transfus'd into each other, if they are both present at the same time; it follows, that when good or evil is plac'd 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. ”

“ But tho' this be confirm'd by undoubted experience, we must understand it with its proper limitations, and must regard the double relation, as requisite only to make one passion produce another. When two passions are already produc'd by their separate causes, and are both present in the mind, they readily mingle and unite, tho' 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 ”

“ They know that his curiosity will precipitate him into the passion they design to raise, and assist the object in its influence on the mind. A soldier advancing to the battle, is naturally inspir'd with courage and confidence, when he thinks on his friends and fellow-soldiers ”

“ We ought to place the object in such particular situations as are proper to encrease the violence of the passion. For we may observe, that all depends upon the situation of the object, and that a variation in this particular will be able to change the calm and the violent passions into each other. ”

“ The connexion is in many respects closer betwixt any two passions, than betwixt any passion and indifference.
When a person is once heartily in love, the little faults and caprice of his mistress, the jealousies and quarrels, to which that commerce is so subject; however unpleasant and related to anger and hatred; are yet found to give additional force to the prevailing passion. 'Tis a common artifice of politicians, when they wou'd affect any person very much by a matter of fact, of which they intend to inform him, first to excite his curiosity ”