概述
若用通俗的語言概述凱恩斯的經濟理論，其要旨一般外行人也能理解。他指出，國民收入可分為兩部分：一部分用於消費，另一部分儲蓄，儲蓄會完全以新資本形式重新回流於經濟體係。因此 儲蓄 = 投資。
（投資乘數）他的理論第一部分是關於投資乘數。他主張，消費與儲蓄的比率主要取決於總收入的規模。換言之，設 m = 消費 / 儲蓄，T 為總收入，則 m = m(T)。由於 儲蓄 = 投資， 我們有 T = 消費 + 儲蓄 = (m(T) + 1) 儲蓄 = (m(T) + 1) 投資。 因此，他主張，繁榮取決於投資的規模。投資增長或減少，繁榮亦隨之增長或減少。（他也主張 m(T) 隨 T 增大而遞減。）k(T) = 1 + m(T) 即為他的投資乘數。
（投資的誘因）他的理論第二部分講什麼會誘發投資。他認為投資者會做某些計算：估算未來投資可能獲得的回報率，並將其與貨幣利率比較。隻有當預期回報率（他稱為「資本邊際效率」）高於貨幣利率時，投資者才會投資。 依此理論，解決失業問題有兩條路：降低利率或提高資本邊際效率。利率可由貨幣當局操作，不過有限製，因為利率必須保持正值。資本邊際效率在凱恩斯看來部分是心理性的，可望透過政府的赤字支出來刺激。在像大蕭條那樣特別糟糕的時期，資本邊際效率接近零，則可能需要兩者並用以緩解失業之禍。
（利率與貨幣）為使上述論述成立，企業家的預期回報率與貨幣利率需彼此獨立。這導致其理論第三部分：利率與貨幣理論。凱恩斯認為貨幣具有三項使其獨特的性質： (a) 貨幣並非普通商品，無法由勞動生產；相反，貨幣可由貨幣當局隨意抽出或放回流通，幾乎無成本。 (b) 貨幣無持有成本。 (c) 貨幣可以立即用來購買任何其他商品（貨幣的流動性）。
凱恩斯主張，項目 (c) 是貨幣利率所代表的客觀價值，且由於 (b) 與 (c)，貨幣利率必為正值。項目 (a) 則較為微妙，它暗示利率可由貨幣當局操縱。貨幣當局可調整流通中的貨幣總量以操縱利率：若流通貨幣量減少，則 (c) 的客觀價值會上升，反之亦然。這就是他對利率與貨幣的理論。
短評
經濟學作為一門學術科目，與物理科學大不相同；在物理中，理論可在受控環境中由反覆取得的實證資料客觀檢驗。在經濟學中，許多看似完全合理的觀察曾帶來宏大承諾，卻很快被遺忘。就整個經濟係統而言，受控且可重複的實驗不可能，且係統複雜到可以輕易向相反方向論證——隻要認真思考，人們總能向一個看似合理的論點投擲曲線球，使其結論完全偏離。可靠的經濟觀察總是可被爭論，而辯論很快會退化為無休止、含糊不清的個人意見。
然而，既有的社會與經濟秩序需要理論來為其基本組織原則辯護。 對於資本主義社會而言，建構一個可接受的理論並不容易。我們的時代是科學時代，因此正當性必須建立在世俗理由與實證資料上。但有需求就會有供給，並且確實有一套理論從無休止的爭論中形成，這就是凱恩斯在書中說的古典理論。
古典理論實質上將每一個經濟問題，不論大小，視為一個可用微分法求解的數學最佳化問題。進一步，發展出了一套係統性的分析方法，將經濟問題視為供給與需求相遇的問題。對一個又一個問題提供了優雅的解答，得出類似結論：自由市場就像一個擺錘，會圍繞由供需原則決定的穩定均衡不斷來回擺動。基本結論是：雖有起伏，但放任體製無論在小尺度或大尺度上，自我調節且本質上是穩定的。 例如，關於就業問題，古典理論的答案如下：令就業人數為 N，單位工資為 W。我們將 N 視為 W 的函數。對雇主而言，工資越低會雇用越多人，故需求函數 N = D(W) 為單調遞減。對勞工而言，工資越高會有越多人願意工作，供給函數為 N = S(W)，單調遞增。這兩個函數會在唯一的 N 值相交，該值即為就業的均衡。 古典理論也對利率問題給出了解答。此處有借款人與放款人：令 r 為利率，M 為可貸款總額。對借款人而言，r 越高則借的越少，因此需求曲線 M = D(r) 單調遞減。對放款人則相反：r 越高則願意提供的貸款越多，因此供給函數 M = S(r) 為單調遞增。這兩個函數會在唯一 r 值相交，該均衡值即為係統的有效利率。
受官方教育體製栽培的古典理論家逐漸滲透並掌控了所有有實際影響力的經濟機構。在與其他競爭理論的較量中，他們的勝利是悄無聲息且徹底的——若深思其過程，令人驚訝。古典理論當然具備相當的智慧，但他們的勝利顯然不僅僅建立在學術上的優勢。他們也並非積極尋求征服世界，因為多數是學術專業人士。雖不乏自負與傲慢，但他們更喜歡與同儕進行風趣的討論，而非麵對現實世界的醜陋。卡爾·馬克思稱他們為製度的辨護者，這稱呼並非無理。既有的社會與經濟秩序迫切需要一套辯護性的理論，而非宣判其死刑的理論，因此這些辯護者獲得了勝利。 到凱恩斯時代，他們的地位與理論已相當根深蒂固。
這便是凱恩斯麵臨的困境：他的理論與古典的就業理論及古典的利率理論（如上所述）相互衝突。對一位相信自己發現了具有根本重要性的學術成果，卻發現新發現挑戰當時既得權威的人而言，向當代學界推銷其理論通常幾乎不可能。新思想難以勝過那些已在舊思想上投入聲望與私利的人。對他而言，唯一的希望在於未來——當舊一代力量隨歲月衰退，新思想逐漸為年輕且承諾較少的人接受時，或許能見曙光。新思想勝出可能需時甚久。不幸的是，「長期來看我們都死了」，對渴望世俗影響力的自我者而言，等待從不是選項。於是凱恩斯踏上了一項幾乎不可能的任務，寫下《就業、利息和貨幣通論》。
凱恩斯最終成功的原因有二：首先，他本身是古典學派體係中最具聲望的人物之一，深入了解古典理論，其學術理論化的天賦也遠超同時代人；其次，他的時代正逢大蕭條，古典理論的威望因現實困境而大為削弱。各國資本主義政府被迫或多或少實際放棄放任政策以避免社會經濟秩序全麵崩潰。某些政策，尤其是新政，需要新的經濟理論，而凱恩斯的理論恰好適配。 他呈現新理論的方式巧妙：把自己的理論表述為對古典理論的改進與推廣。在一個人人都既得其利的世界裡，若主張蘋果比桃子好，應砍掉所有桃樹改植蘋果樹，這樣的說法絕對行不通。相反，他主張隻要做出與以往稍有不同的處理，就能在舊的桃樹上長出蘋果。 正如凱恩斯在其書前言所指出：「這裡費力表達的觀念其實極為簡單，應當顯而易見。難點不在於新觀念，而在於擺脫舊觀念——這些舊觀念對於像我們大多數人一樣受過教育的人，已蔓延到心靈的每一角落。」
(I) 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 are divided into two part: the part spent for CONSUMPTION and the part that is SAVED, which 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 depend 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 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 would 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 the 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 can 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 verse. This is his theory on interest rate and money.
(Remarks) These, in any rate, appear to be rather sound observations. Economics as an academic subject is quite different from the subject of physical sciences, where theories could be objectively tested by either empirical data repeatedly obtained in controlled environment. In economics, numerous observations, all appeared perfectly sound at times, had come with great promises then quickly passed into oblivion. As far as the entire economic system is concerned, controlled repeatable empirical experiments are out of question and one can easily argue things in opposite directions because the system is so complicated that, if one thinks hard enough, he could always throw a curve ball into a reasonably sound argument to completely swerve the directions of its conclusion. Sound economic observations are always debatable and, very quickly, debate would degenerate into endless mumble-jumbles of personal opinion.
Nonetheless, an established social and economic order is in need of a theory to defend its fundamental organizational principles.
For the capitalist society, to formulate an acceptable theory has been hard. Our era is the era of science so our justification has to be built on earthly reasons and empirical data. But where there is a demand then there will be a meeting supply, and a theory indeed emerged from the endless mumble-jumbles, a theory that is referred to by Keynes as the classical theory in his book. The classical theory, in essence, treated every economic problem, large or small, as a mathematical problem of optimization, which can be solved in principle by differentiable calculus. A systematic way of analysis was further developed to treat economic problems as problems in which supply meets demand. Elegant solutions were offered to problem after problem, with similar conclusions that everything in the laissez faire is like a pendulum, swing forever back and forth around a stable equilibrium dictated by the principle of supply meets demand. The fundamental conclusion: we would have ups and downs but the system of laissez faire, left alone, in both small and large scale, is intrinsically self-adjusting therefore stable.
For instance, concerning the problem of employment, the answer offered by the classical theory is as follows: let the number of people employed as $N$, and the unit wage of labor as $W$. We regard $N$ as a function of $W$. For an employer, he would hire more people if the wage is lower, and this is to say that the demanding function, $N = D(W)$ is monotonically decreasing. For the laborer, on the other hand, more would be forthcoming if the wage is higher, and this is the supply function we denote as $N = S(W)$, an increasing function. These two functions would meet at a unique value of $N$, and this value is the equilibrium for employment.
The classical theory also offered an answer to the problem of interest rate. Here we have borrowers and lenders: Let $r$ be the interest rate and $M$ be the total money lend. For the borrowers, if $r$ is high then they would borrow less, so the demand schedule $M = D(r)$ is monotonically decreasing. For lenders it is the opposite. If $r$ is higher then more would be forthcoming to lend, so the supply function $M = S(r)$ is monotonically increasing. These two functions would again meet at a unique value of $r$, and this equilibrium value of interest is the effective interest rate of the system.
Classical theorists breaded in the official educational system, gradually infested and conquered all established economic institutions of practical influence. Against other competing theories, their victory was quiet, complete, and, if one reflects a little on it, stunning. The classical theory is for sure not without substantial intellectual merit, but their victory was clearly not built solely on intellectual merit. Nor did they actively seek to conquer the world, for these were mostly academic professionals. Though not without their own share of ego and arrogance, they would prefer far more in the delight of a witty chat with their colleagues than in facing the ugliness of the real world. Karl Marx called them the apologist of the system. Justly so. But the established social and economic order was in desperate need of an apologetic theory, not one that pronounced its death sentence. So the apologists won.
By the time of Keynes, their position and their theory had been well-entrenched.
So here was the trouble for Keynes: his theory is in conflict with the classical theory of employment and the classical theory of interest rate (as outlined in the above). For someone who believes that he has made academic discoveries of fundamental importance but his new findings are against the establishment of his time, to sell his theory to his contemporaries is usually a virtue impossibility. New ideas could never win over the ones whose prestige and self-interest have been deeply invested in the old. The only hope for him was in the future, when the power of the current authority wanes with the pass of the old generation and the new ideas are gradually accepted by the younger and less committed minds. It might take a long time for new ideas to win out. Unfortunately, ``in the long run we are all dead", and for an egoistic soul craving worldly influences, waiting was never an option. Hence Lord Keynes was set for the mission impossible, with the book ``The general theory of employment, interest and money".
The reasons why Keynes was ultimately successful in this impossible deed lied in two things. First he was among the most prestige member of the classical establishment. He knew the classical theory inside out and his talent in academic theorization was far superior to his contemporaries. Second his time was the time of great depression, in which the prestige of the classical theory was mostly diminished by hard reality. The capitalist governments around the world were forced to practically abandon laissez faire one way or the other to prevent a complete melt-down of social and economic order. Some of their practices, in particular, the new deal, was in need of a new economic theory and what Keynes presented was a perfect fit.
The way he presented his new theory was ingenious. He presented his theory as improvement and generalizations over the classical theory. In a world everyone was vested in peach, to argue apple is better therefore one should abolish all peach tree and plant apple tree would be an absolute no-go. Instead, he claimed that by doing something somewhat different from before, one could grow apple from the old peach tree.
As pointed out by Keynes himself in the preface of his book: ``The idea which are here expressed so laboriously are extremely simple and should be obvious. The difficulty lies, not in new ideas, but in escaping from the old ones, which ramify, for those brought up as most us have been, into every corner of our mind."
