T 3.2.5.9

All this is the effect of the natural and inherent principles and passions of human nature; and as these passions and principles are inalterable, it may be thought, that our conduct, which depends on them, must be so too, and that 'twou'd be in vain, either for moralists or politicians, to tamper with us, or attempt to change the usual course of our actions, with a view to public interest. And indeed, did the success of their designs depend upon their success in correcting the selfishness and ingratitude of men, they wou'd never make any progress, unless aided by omnipotence, which is alone able to new-mould the human mind, and change its character in such fundamental articles. All they can pretend to, is, to give a new direction to those natural passions, and teach us that we can better satisfy our appetites in an oblique and artificial manner, than by their headlong and impetuous motion. Hence I learn to do a service to another, without bearing him any real kindness; because I forsee, that he will return my service, in expectation of another of the same kind, and in order to maintain the same correspondence of good offices with me or with others. And accordingly, after I have serv'd him, and he is in possession of the advantage arising from my action, he is induc'd to perform his part, as foreseeing the consequences of his refusal.