[...] In standard introductory economics textbooks, money is commonly said to perform four functions: (1) It is a medium of exchange that facilitates the exchange of qualitatively different commodities. (2) It is a means of payment or transaction settlement, the thing with which you settle a debt, and usually legally defined as such. (3) It is a store of value; you can hold it as an asset in the form of abstract or potential purchasing power. (4) It is a unit of account; i.e., the standard unit by which all "economic" values are calculated and compared.

[...] classical and neoclassical economic analysis suggests that all of these functions from an original and primary function, (1) medium of exchange. [...] However, when examined in a more historically sensitive--and analytically adequate--manner, it is clear that as capitalism evolved, money became central primarily because of its key function as a standardized unit of account, which at least in capitalism is the most important role money plays. [...] as a unit of account it represents an abstract claim on or in circulation as a whole. Money measures and stores abstract purchasing power, and transports it through space and time. [...]