comparison with what we consider to be normal human psychology, and that the rules of justice are so "obvious" and "necessary" an invention that they can still be deemed "natural" to the human species. Hume next devotes an important and lengthy section to two questions: First, how is the social convention of justice established? And second, why do we invest the rules of justice with moral significance? His answer to the first question begins with our need for society. Humans are not strong, skilled, or secure enough to meet our needs alone, and only society can offer additional labour force, specialisation, and mutual aid—all important advantages of society learned of through growing up in families. But this necessary social union is threatened both by human selfishness (or rather "confin'd generosity") and by the scarcity and instability of external goods. And since our uncultivated natural affections cannot overcome these obstacles (we see nothing wrong with having a normal amount of selfishness and generosity), it is left to our reason and self-interest to find a solution: through "a general sense of common interest" that is "mutually expressed" and known to everybody, we gradually develop a social convention for the stabilizing and safeguarding of external goods, with improved compliance and stronger social expectations feeding into each other, a process Hume compares to the development of languages and currency. He insists that this convention is not a promise, famously illustrating the point with the example of two men agreeing to row a boat together, simply from a sense of mutual advantage rather than from any promise. And as justice is defined in terms of such a convention, so too the related concepts of "property, or right, or obligation" can mean nothing in its absence. Since the chief obstacle to society (our selfishness, especially our insatiable acquisitiveness) is in fact the very motive responsible for society, the growth of social order depends less on our moral qualities than on our intellectual qualities. But since stabilizing external goods is such a "simple and obvious" rule, the convention is established with little delay, so that "the state of nature" is a "mere philosophical fiction"—not very realistic but useful for theorizing. Similarly edifying, "the golden age" (a fictional time of superabundant resources and universal brotherly love) helps shed light on the origins of justice: were it not for certain non-ideal circumstances (selfishness, limited generosity, resource scarcity, resource instability),