  A theoretical self-sustainable economic model is established based on the
fundamental factors of production, consumption, reservation and reinvestment,
where currency is set as a unconditional credit symbol serving as transaction
equivalent and stock means. Principle properties of currency are explored in
this ideal economic system. Physical analysis reveals some facts that were not
addressed by traditional monetary theory, and several basic principles of ideal
currency are concluded: 1. The saving-replacement is a more primary function of
currency than the transaction equivalents; 2. The ideal efficiency of currency
corresponds to the least practical value; 3. The contradiction between constant
face value of currency and depreciable goods leads to intrinsic inflation.
