The mutual dependence of men is so great, in all societies, that scarce any human action is entirely compleat in itself, or is performed without some reference to the actions of others, which are requisite to make it answer fully the intention of the agnet. The poorest artificer, who labours alone, expects at least the protection of the magistrate, to ensure him the enjoyment of the fruits of his labour. He also expects, that, when he carries his goods to the market, and offers them at a reasonable price, he shall find purchasers; and shall be able, by the money he acquires, to engage others to supply him with those commodities, which are requisite for his subsistence. In proportion as men extend their dealings, and render their intercourse with others more complicated, they always comprehend, in their schemes of life, a greater variety of voluntary actions, which they expect, from the proper motives, to co-operate with their own.
Hume, Enquiry Concerning Human Understanding, VIII, Part I, par. 17. Hume goes on to argue, plausibly, that this requires that human actions be relatively uniform and predictable; although it's questionable that it requires it to be as uniform and predictable as Hume thinks it does.
