凯恩斯的宏观经济学：背景
蒋闻铭
作为一门学科，经济学与物理科学有本质的不同：在物理科学中，理论可以在受控环境下通过反复获得的经验数据进行客观检验；而在经济学中，许多曾经看似完全合理的观察与理论，往往一度承载着巨大的期望，却又很快被遗忘，湮没于历史之中。
然而，现行的社会经济秩序，需要一套完整的理论来为他的最基本的组织原则作辩护。想要为资本主义的经济制度，构建一套令人信服的理论，很不容易。在一个以科学为标准的时代，这种辩护必须建立在现实理由与经验数据之上。不过，有需求就会有供给，在纷繁杂乱、众说纷纭的探索之中，经济学界逐渐形成了一套完整的理论，这便是凯恩斯口中的“古典理论”。
古典理论，将一切经济问题——无论大小——都视作为原则上可以通过微积分的方法加以解决的优化问题。所有的经济问题，在这些人眼里，都可以被理解成供给与需求的相互作用， 不但容易理解，而且他们有一套系统化的分析方法，对所有的问题给解答。他们的主要结论， 是在自由放任（laissez-faire）的体制下，经济运行如同一个钟摆，围绕着由供求关系所决定的稳定均衡点不断来回摆动。所以自主独立的市场经济体系，虽然会有起伏波动，但在不加干预的情况下，无论在微观还是宏观层面，都具有内在的自我调节能力，因此总体上是稳定的。
例如在就业问题上，古典理论给出的解释如下：设就业人数为 N，劳动的单位工资为 W，我们将 N 视为 W的函数。对于雇主而言，如果工资越高，他愿意雇佣工人就会越少，这意味着需求函数 N=D(W)是单调递减的；而对于劳动者来说，工资越高，愿意提供的劳动力就越多，这便是供给函数 N=S(W)，它是一个递增函数。这两条函数曲线将在某一个唯一的 N 值处相交，而这一点便是就业的均衡。
古典理论也对利率问题给了类似的解释。在这里，我们考虑借款者与放款者：设利率为 r，借贷资金总量为 M。对于借款者而言，利率越高，借款意愿越低，因此需求函数 M=D(r) 是单调递减的；而对于放款者来说则正相反，利率越高，愿意贷出的资金就越多，因此供给函数 M=S(r) 是单调递增的。这两条函数同样会在某一个唯一的 r值处相交，而这一均衡利率便是整个经济体系中的实际利率。
古典理论家在官方教育体系中不断滋生，逐步渗透并占据了几乎所有具有实际影响力的既有经济机构。与其他理论相比，他们的胜利是悄然发生的，却很是彻底，细想起来甚至有些令人震惊。
这个理论无疑并非没有重要的智识价值，但他们的胜利，显然并不只是建立在纯粹的学术成就之上。这些经济学家们，本身也并未有意去征服世界。他们的大多数，尽管也不乏自负与傲慢，但与直面现实世界的复杂与丑陋相比，更乐于沉浸在与同行之间机智而优雅的学术交谈之中。
Karl Marx 称他们为制度的辩护士（apologist），这一评价是恰如其份。然而，现实的社会与经济秩序迫切需要的，是一种为自身辩护的理论，而不是宣判这种经济秩序必然灭亡的理论。于是辩护者取得了完全的胜利。到了凯恩斯的时代，他们的地位与理论在经济学界，都已是根深蒂固。
凯恩斯所面临的困境是这样的：他的理论与古典就业理论以及古典利率理论（如前所述）发生了直接冲突。对于一个自信已经取得了具有根本意义的学术发现的人来说，当这些新见解与其时代的主流学术理论相抵触时，要让同时代的人接受自己的理论，几乎不可能。
新的思想，没有可能会说服那些声望与既得利益早已深深绑定在旧理论之中的人们。 他唯一的希望在未来——当既有权威随着旧一代人的消逝而逐渐衰退时，新思想才可能被年轻一代、那些尚未深度投入既有体系的人所逐步接受。这个过程往往需要很长时间。然而，不幸的是，“从长远看，我们都会死”（in the long run， we are all dead）。对于一个渴望现实影响力、带有强烈自我意识的人而言，等待从来都不是选项。
因此，凯恩斯给自己找了一项近乎不可能完成的任务：撰写《The General Theory of Employment, Interest and Money》。
凯恩斯之所以最终能够成功，主要有两个原因。
第一，他本身就是古典学派中最具声望的人物之一。他对古典理论了如指掌，而且他构建理论的能力远远超过同时代的经济学者。第二，他所处的时代正值经济大萧条，现实的冲击极大地削弱了古典理论的威望。世界各国的资本主义政府，为了防止社会与经济秩序的全面崩溃，不得不以各种方式在实践上放弃自由放任的政策。其中的一些做法，尤其是New Deal，迫切需要新的经济理论做支撑，而凯恩斯的理论适逢其会，恰好提供了这个理论支撑。
他提出新理论的方式也极为高明。他并没有将自己的理论表述为对古典理论的彻底否定，而是将其包装为一种“改进”与“推广”。在一个人人都种桃树的世界里，若有人宣称苹果更好，因此应当砍掉所有桃树改种苹果树，必然不会被接受；相反，他声称只需稍作不同的处理，便可以在原有的桃树上结出苹果。
正如凯恩斯在其著作《The General Theory of Employment, Interest and Money》序言中所说：“这里费尽周折所表达的思想，其实极为简单，本应不言自明。困难不在于提出新思想，而在于摆脱旧思想——对于我们大多数人来说，这些旧思想早已根深蒂固，盘根错节，渗透于我们心灵的每一个角落。”
前一篇：凯恩斯的宏观经济学：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."
