凱恩斯的宏觀經濟學：背景
蔣聞銘
作為一門學科，經濟學與物理科學有本質的不同：在物理科學中，理論可以在受控環境下通過反複獲得的經驗數據進行客觀檢驗；而在經濟學中，許多曾經看似完全合理的觀察與理論，往往一度承載著巨大的期望，卻又很快被遺忘，湮沒於曆史之中。
然而，現行的社會經濟秩序，需要一套完整的理論來為他的最基本的組織原則作辯護。想要為資本主義的經濟製度，構建一套令人信服的理論，很不容易。在一個以科學為標準的時代，這種辯護必須建立在現實理由與經驗數據之上。不過，有需求就會有供給，在紛繁雜亂、眾說紛紜的探索之中，經濟學界逐漸形成了一套完整的理論，這便是凱恩斯口中的“古典理論”。
古典理論，將一切經濟問題——無論大小——都視作為原則上可以通過微積分的方法加以解決的優化問題。所有的經濟問題，在這些人眼裏，都可以被理解成供給與需求的相互作用， 不但容易理解，而且他們有一套係統化的分析方法，對所有的問題給解答。他們的主要結論， 是在自由放任（laissez-faire）的體製下，經濟運行如同一個鍾擺，圍繞著由供求關係所決定的穩定均衡點不斷來回擺動。所以自主獨立的市場經濟體係，雖然會有起伏波動，但在不加幹預的情況下，無論在微觀還是宏觀層麵，都具有內在的自我調節能力，因此總體上是穩定的。
例如在就業問題上，古典理論給出的解釋如下：設就業人數為 N，勞動的單位工資為 W，我們將 N 視為 W的函數。對於雇主而言，如果工資越高，他願意雇傭工人就會越少，這意味著需求函數 N=D(W)是單調遞減的；而對於勞動者來說，工資越高，願意提供的勞動力就越多，這便是供給函數 N=S(W)，它是一個遞增函數。這兩條函數曲線將在某一個唯一的 N 值處相交，而這一點便是就業的均衡。
古典理論也對利率問題給了類似的解釋。在這裏，我們考慮借款者與放款者：設利率為 r，借貸資金總量為 M。對於借款者而言，利率越高，借款意願越低，因此需求函數 M=D(r) 是單調遞減的；而對於放款者來說則正相反，利率越高，願意貸出的資金就越多，因此供給函數 M=S(r) 是單調遞增的。這兩條函數同樣會在某一個唯一的 r值處相交，而這一均衡利率便是整個經濟體係中的實際利率。
古典理論家在官方教育體係中不斷滋生，逐步滲透並占據了幾乎所有具有實際影響力的既有經濟機構。與其他理論相比，他們的勝利是悄然發生的，卻很是徹底，細想起來甚至有些令人震驚。
這個理論無疑並非沒有重要的智識價值，但他們的勝利，顯然並不隻是建立在純粹的學術成就之上。這些經濟學家們，本身也並未有意去征服世界。他們的大多數，盡管也不乏自負與傲慢，但與直麵現實世界的複雜與醜陋相比，更樂於沉浸在與同行之間機智而優雅的學術交談之中。
Karl Marx 稱他們為製度的辯護士（apologist），這一評價是恰如其份。然而，現實的社會與經濟秩序迫切需要的，是一種為自身辯護的理論，而不是宣判這種經濟秩序必然滅亡的理論。於是辯護者取得了完全的勝利。到了凱恩斯的時代，他們的地位與理論在經濟學界，都已是根深蒂固。
凱恩斯所麵臨的困境是這樣的：他的理論與古典就業理論以及古典利率理論（如前所述）發生了直接衝突。對於一個自信已經取得了具有根本意義的學術發現的人來說，當這些新見解與其時代的主流學術理論相抵觸時，要讓同時代的人接受自己的理論，幾乎不可能。
新的思想，沒有可能會說服那些聲望與既得利益早已深深綁定在舊理論之中的人們。 他唯一的希望在未來——當既有權威隨著舊一代人的消逝而逐漸衰退時，新思想才可能被年輕一代、那些尚未深度投入既有體係的人所逐步接受。這個過程往往需要很長時間。然而，不幸的是，“從長遠看，我們都會死”（in the long run， we are all dead）。對於一個渴望現實影響力、帶有強烈自我意識的人而言，等待從來都不是選項。
因此，凱恩斯給自己找了一項近乎不可能完成的任務：撰寫《The General Theory of Employment, Interest and Money》。
凱恩斯之所以最終能夠成功，主要有兩個原因。
第一，他本身就是古典學派中最具聲望的人物之一。他對古典理論了如指掌，而且他構建理論的能力遠遠超過同時代的經濟學者。第二，他所處的時代正值經濟大蕭條，現實的衝擊極大地削弱了古典理論的威望。世界各國的資本主義政府，為了防止社會與經濟秩序的全麵崩潰，不得不以各種方式在實踐上放棄自由放任的政策。其中的一些做法，尤其是New Deal，迫切需要新的經濟理論做支撐，而凱恩斯的理論適逢其會，恰好提供了這個理論支撐。
他提出新理論的方式也極為高明。他並沒有將自己的理論表述為對古典理論的徹底否定，而是將其包裝為一種“改進”與“推廣”。在一個人人都種桃樹的世界裏，若有人宣稱蘋果更好，因此應當砍掉所有桃樹改種蘋果樹，必然不會被接受；相反，他聲稱隻需稍作不同的處理，便可以在原有的桃樹上結出蘋果。
正如凱恩斯在其著作《The General Theory of Employment, Interest and Money》序言中所說：“這裏費盡周折所表達的思想，其實極為簡單，本應不言自明。困難不在於提出新思想，而在於擺脫舊思想——對於我們大多數人來說，這些舊思想早已根深蒂固，盤根錯節，滲透於我們心靈的每一個角落。”
前一篇：凱恩斯的宏觀經濟學：概述
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 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 subjective opinions.
Nonetheless, an established social and economic order needs 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 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 will hire more people if the wage goes 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, 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 breed 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. This 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. Consequently, 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 conflicts 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 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 influence, 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 peaches; 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."
