To summarize the known facts briefly, money is transferable freedom against a limited choice of goods. It guarantees this freedom abstractly, by means of an opportunity for exchange, on a quantitatively limited basis, leaving open questions of when, with whom, for which object and under what conditions the person possessing the money will carry out the exchange.

DEFINE: Defines money not as a physical object but as abstract, transferable freedom constrained to a limited range of goods, with exchange opportunities deferred rather than realized.

Niklas Luhmann, Trust and Power, loc. 210