The philosophy of money is not empty theorizing. This is the key factor that determines the limits of freedom and equality of people in a society with the money economy. The subject of money has fascinated wise man from the time of Aristotle to the present day because it is so full of mystery and paradox. The definition of money in contemporary economics is that money is something used to effect exchange. Money, Banking, and Monetary Policy.
Money is whatever money does.
All that performs the functions of money is money. The large literature concerned with economic methodology is of uneven quality, and there is striking disparity between the quality of the economic contributions and of the methodological reflections of major economists. People direct all of their capabilities towards the acquisition of money, as if this were an end, and no effort were to be spared to attain this end. Volume 1, trans. Pelican Marx Library.