Finance is a field that deals with the allocation of assets and liabilities over time under conditions of certainty and uncertainty. Finance also applies and uses the theories of economics at some level. Finance can also be defined as the science of money management. A key point in finance is the time value of money, which states that purchasing power of one unit of currency can vary over time. Finance aims to price assets based on their risk level and their expected rate of return. Finance can be broken into three different sub-categories: public finance, corporate finance and personal finance.