As most philosophers regarded money with a certain distrust or even contempt, the question of the nature of money was not addressed from a philosophical perspective until G. Simmel wrote The Philosophy of Money (1900). Even 19th-century thinkers clung to the idea of money as a commodity, or, for Marx, as a "mask of value". This idea is rather misleading, as money, in comparison with commodities, always flows in the opposite direction, is not consumed but rather circulates, and can be represented by fewer and fewer material tokens, by fiduciary money, or even by invisible computer bytes. On the other hand, it appears to be a sort of institutionalised mutual trust, the validity of which is attested to daily by the willingness (or unwillingness) of citizens to accept it as payment. Though the monetary economy has a natural tendency to extend itself over as many commodities as possible, money itself must, paradoxically, be strictly excluded from free market competition: its production is monopolised or at least regulated. The same is true for the agents of social control: if to bribe a judge, mayor or police officer were to become an efficient alternative to buying, nobody could be expected to sell. Money seems to be almost a genial social invention, facilitating daily life, the division of labour, and collaboration, and promoting a kind of civic egality. Its only drawback is that its validity depends on the general level of mutual trust among citizens. Thus the views of Socrates and of Adam Smith, both of whom saw money as somehow related to the morals and trustworthiness of a society, may be confirmed, albeit on different bases.