凯衍斯的交易基本方程式

(3)凯衍斯的交易基本方程式

凯衍斯的《货币论》(A Treatise on Money)出世以后，轰动一时。崇拜他的人至誉为亚丹·斯密《原富》(AdamSmith，Wealth of Nations)以后稀有的名著，然其书艰涩难读，即有名的经济学者师丹普(Josia Stamp)也说，有时候要费半小时才能看懂一面。理论的高深和复杂可以想见。他所用的名词和寻常的意义很多不同。第一，他把存款先分为收入存款(Income deposits)与商业存款(Business deposits)两种，前者为个人留存备用的银行存款与手存现金，后者为工商业界暂存备用的银行存款与库存现金。两者并称为现金存款(Cash deposits)。此外为储蓄存款(Saving deposits)，为留存投资的银行存款。第二，货币的购买力是人们为消费起见用其货币收入(Money income)购买货物和劳役的货币之力量。^[40]第三，收入(Income)是社会的货币收入，或生产要素的所得(Earnings of factors of production)，或生产成本(Cost of production)。第四，利润(profit)是生产成本与实际卖价(Actualsale-proceeds)之差，不在收入之内。第五，储蓄(Savings)是个人的货币收入与当时消费者的货币支出(Money-expenditure on current consumption)之差。第六，投资(Investment)是储蓄与利润之和。

全书最精彩的地方在交易的基本方程式(Fundamental Equations of Exchanges)一章。那里有10个公式，现在列举最初两个如下:

P=price level of liquid-consumption goods.

O=total output of goods per same cost of production.

E=total money-income or earnings of the community in a unit of time.

I'=the part of income earned by the production of investment goods.

S=the amount of savings.

R=the volume of liquid consumption-goods and savings flowing on to the market and purchased by consumers.

W=the rate of earnings per unit of output.

10个公式之中，有一个共通的因素，就是I'－S。这个因素是凯衍斯学说的中心。^[41]他批评诸家学说，如马夏律、披古、陶昔格(Taussig)、费雪、嘉塞尔、何屈理，说是都不曾贯彻到底，只有维克霰(KuntWicksell)的利息与物价(Geldzins und Güterpreise，1898)应受一般人更大之注意，和他的《货币论》极相近。^[42]

他把物价平准分为两部分:一是消费物的物价平准，二是资本物的物价平准。在平衡状况中，都要受生产成本所决定。但问题的中心是:消费物的物价平准实际上是否与其生产成本相等，纯视乎用在消费的收入是否与其消费物的生产量成正比例，换一句话说，纯视乎收入划分为储蓄和消费两部分是否与生产成本划分为资本物成本和消费物成本两部分相等。

“The price-level of output as a whole during any period ismade up of two components—the price-level of the goods coming forward for consumption and the price-level of the goods added to the stock of capital.”

“In conditions of equilibrium，both these price-levels are determined by the money cost of production，or，in other words，by the money-rate of efficiency-earnings of the factors of production.”

“The question whether the price-level of the goods which are consumed is in fact equal or unequal to their cost of production，depends on whether the proportion of income of the community which is spent on consumption is or is not the same as the proportion of the output of the community which takes the formof the goods so consumed;in other words，on whether the division of income between savings and expenditure on consumption is or is not the same as the division of the cost of production of output between the cost of goods which are added to capital and the cost of goods which are consumed.”^[43]

如此，消费物的物价平准(即货币购买力的倒数)超过或不及其生产成本，在乎储蓄数量不及或超过新投资的生产成本(即加在资本的货物)。因此，如果储蓄数量超过投资成本，则消费物生产者必受损;如果投资成本超过储蓄数量，则消费物生产者必获利。

“Thus the price-level of the goods which are consumed(i.e.，the inverse of the purchasing power ofmoney)exceeds or falls short of their cost of production，according as the volume of savings falls short or exceeds the cost of production of new investment(i.e.，of the goods which are added to the stock of capital).Hence，if the volume of savings exceeds the cost of investment，the producers of the goods which are being consumed make a loss;and if the cost of investment exceeds the volume of savings，theymake a profit”.^[44]

他的结论是I'－S要为正数，物价才能上涨，工商业才能繁荣，否则物价下落，工商业必定萧条。其余8个公式也是如此。