In economics, money illusion, or price illusion, refers to the tendency of people to think of currency in nominal, rather than real, terms. In other words, the numerical/face value (nominal value) of money is mistaken for its purchasing power (real value) at a previous point in the general price level (in the past). This is false, as modern fiat currencies have no intrinsic value and their real value is derived from all the underlying value systems in an economy, e.g., sound government, sound economics, sound education, sound legal system, sound defence, etc. The change in this real value over time is indicated by the change in the Consumer Price Index over time.