A pipe through which water flows has capacity, but the power that moves the water lies elsewhere. The same is true of money. The monetary unit is merely the conduit through which purchasing power flows, such purchasing power lying in the commodities or values exchanged. Therefore the student of money must be careful not to fall into the error of thinking that money has purchasing power. Things are purchasable only with things. This unchanging law is just as operative under a monetary economy as under a barter economy.