"Money represents, after all, exchange value par excellence, and thereby stands opposed to all other commodities and their use values. Money assumes an independent and external power in relation to exchange because, as the universal equivalent, it is the very incarnation of social power. This social power, furthermore, can be appropriated and used by private persons. The significance of this has now to be worked out. Money permits the separation of sales and purchases in space and time."
--David Harvey, The Limits to Capital, pg 245