凯恩斯的宏观经济学：概述
蒋闻铭
如果用通俗的语言来表达，John Maynard Keynes的经济理论其实相当直观，普通人也很容易理解。
他观察到，一个社会中人们的收入可以分为两部分：一部分用于消费（CONSUMPTION），另一部分则被储蓄（SAVING）下来。通过银行体系、股票市场以及其他各种投资渠道，储蓄部分会被完全返送回经济体系之中，转化为新的投资资本。
因此可以得到一个基本关系：
储蓄 = 投资（SAVING = INVESTMENT）
他接下来的理论，大致分成了三部分
第一部分：投资乘数（investment multiplier）。
既然储蓄等于投资，那么用T代表总收入，
T=总收入=消费+储蓄=(m(T)+1)⋅储蓄=(m(T)+1)⋅投资
因此，他认为，经济的繁荣程度（即总收入）取决于投资（INVESTMENT）的规模：投资增加，收入随之增加；投资减少，收入也随之下降。
他还指出，m(T) 是 T的一个递减函数。而
k(T)=1+m(T)
则被称为他的投资乘数（investment multiplier）。
第二部分：投资的诱因（Inducement of Investments）
他理论的第二个组成部分，是关于什么因素促使投资发生。
他认为，投资者在决策时会估计某项投资在未来可能获得的收益率，并将其与货币利率（money-interest rate）进行比较。只有当他们预期的收益率——也就是所谓的 “资本边际效率” （the marginal efficiency of capital）——高于利率时，才会进行投资。
按照这一理论，解决失业问题有两种途径：一是降低利率，二是提高资本边际效率。
利率可以由货币当局加以调控，但这种调控是有限度的，因为利率必须保持为正值。而资本边际效率，在John Maynard Keynes看来，具有一定的心理性质，因此可以通过政府的赤字支出（deficit spending）来加以刺激。
在经济极度萧条的时期，例如Great Depression，当资本边际效率接近于零时，往往需要同时采取这两种手段，才能缓解失业这一严重问题。
第三部分：利率与货币 (Interest rate and Money)
要使上述分析成立，一个至关重要的前提是：企业家所预期的投资回报率与货币利率必须是相互独立的。这便引出了该理论的第三个组成部分——利率与货币理论。按照John Maynard Keynes的观点，货币具有三项使其独特的性质：
(a) 货币并非普通商品，因为它不能通过劳动生产出来；相反，货币当局可以几乎不花成本地，随意将其投放到流通中或从流通中抽回。
(b) 货币没有持有成本。
(c) 货币可以立即用于购买任何其他商品（即货币的流动性）。
凯恩斯认为，第(c)项体现了货币利率所代表的客观价值；而由(b)与(c)两项可知，货币利率必须为正。至于(a)项则更为微妙，它意味着利率可以被货币当局所操控。货币当局可以通过调节流通中的货币总量来影响利率：例如，当流通中的货币总量减少时，第(c)项所体现的客观价值便会上升，反之亦然。这便是他的利率与货币理论。
An Overview:
The gist of Keynes's economic theory, if spelled in plain English, is one that a layman could easily understand.
He observed that population's income is divided into two parts: the part spent for CONSUMPTION and the part that is SAVED. Through banking system, stock market and other investment avenue, the saved part is completely recirculated back into the economic system as new capital. Hence
SAVING = INVESTMENT.
(Investment Multiplier) The first component of his theory is on the investment multiplier. He argued that the ratio of consumption vs saving depends mainly on the size of the total income. This is to say that let m = COMSUMPTION / SAVING, and T be the total income, then m = m(T). Because
SAVING = INVESTMENT,
we have
T = Consumption + Saving = (m(T) + 1) Saving = (m(T) + 1) Investment.
Hence, he claimed, that prosperity (total income) depends on the size of INVESTMENT. It grows and drops as INVESTMENT grows and drops. He also argued that m(T) is a decreasing function of T. k(T) = 1 + m(T) is his investment multiplier.
(Inducement of Investments) The second component of his theory is on what induces investment. He reckoned that certain calculations are made by investors: they would estimate the rate of return they are likely to acquire in the future for their intended investment and compare it with the money-interest rate. He will invest only if he expects a return rate (which is called under the name ``the marginal efficiency of capital") that is higher than the money-interest rate.
According to this theory then, there are two ways one could address the issue of unemployment: to reduce the interest rate and to raise the marginal efficiency of capital. The interest rate can be manipulated by monetary authority, though this is not without its limit, for the interest rate will have to stay positive. The marginal efficiency of capital, according to Keynes, is partly psychological therefore can hopefully be stimulated by deficit spending of the government. In particularly bad times, like that of the great depression, in which the marginal efficiency of capital was close to zero, one would need both to alleviate the evil of unemployment.
(Interest rate and Money) For the above to make sense it is critically important that the expected return rate of the entrepreneur and the money-interest rate are independent. This then led to the third component of this theory: the theory of interest rate and money. Money, according to Keynes, has three properties that make it unique.
(a) Money is not an ordinary commodity in the sense that it could not be produced by labor. It, on the other hand, can be extracted from and put back into circulation by the monetary authority at will with virtually no cost.
(b) Money has no carrying cost.
(c) Money can be used instantly to buy any of the other commodity. (Liquidity of money).
Keynes argued that item (c) is the objective value represented by the money interest rate, and by item (b) and (c) money-interest must be positive. Item (a) is tricky, it implies that interest rate can be manipulated by monetary authority. The monetary authority can adjust the total quantity of money in circulation to manipulate the interest rate: observe that if total quantity of money in circulation is reduced, then the objective value of (c) would increase and vice versa. This is his theory on interest rate and money.
