Notes from the blog SOCIAL DEMOCRACY FOR THE 21ST CENTURY: A REALIST ALTERNATIVE TO THE MODERN LEFT.
Author: Lord Keynes(LK)
Website:https://socialdemocracy21stcentury.blogspot.com/p/the-following-is-index-to-my-posts-on.html
向LK表达敬意。
凯恩斯在整部《通论》中只用了 3 次“动物精神”这个词，而且都是在第 12 章的末尾，但这个概念似乎引起了相当大的混乱。
“Even apart from the instability due to speculation, there is the instability due to the characteristic of human nature that a large proportion of our positive activities depend on spontaneous optimism rather than on a mathematical expectation, whether moral or hedonistic or economic. Most, probably, of our decisions to do something positive, the full consequences of which will be drawn out over many days to come, can only be taken as a result of animal spirits—of a spontaneous urge to action rather than inaction, and not as the outcome of a weighted average of quantitative benefits multiplied by quantitative probabilities. Enterprise only pretends to itself to be mainly actuated by the statements in its own prospectus, however candid and sincere. Only a little more than an expedition to the South Pole, is it based on an exact calculation of benefits to come. Thus if the animal spirits are dimmed and the spontaneous optimism falters, leaving us to depend on nothing but a mathematical expectation, enterprise will fade and die;—though fears of loss may have a basis no more reasonable than hopes of profit had before.”
“It is safe to say that enterprise which depends on hopes stretching into the future benefits the community as a whole. But individual initiative will only be adequate when reasonable calculation is supplemented and supported by animal spirits, so that the thought of ultimate loss which often overtakes pioneers, as experience undoubtedly tells us and them, is put aside as a healthy man puts aside the expectation of death. ”
其实凯恩斯的定义非常明确：一种使人类自发行动而非无所作为的驱动力。这一概念应当被详细地阐释为：人类能够并且确实在不确定性下采取行动，无论是面对真实的还是知觉的不确定性。（凯恩斯强调的不确定性意味着根本不存在未来的概率分布。）
“By ‘uncertain’ knowledge, let me explain, I do not mean merely to distinguish what is known for certain from what is only probable. The game of roulette is not subject, in this sense, to uncertainty; nor is the prospect of a Victory bond being drawn. Or, again, the expectation of life is only slightly uncertain. Even the weather is only moderately uncertain. The sense in which I am using the term is that in which the prospect of a European war is uncertain, or the price of copper and the rate of interest twenty years hence, or the obsolescence of a new invention, or the position of private wealth owners in the social system in 1970. About these matters there is no scientific basis on which to form any calculable probability whatever.”
预期是主观的，投资决策受动物精神驱使的（这并不是理性预期假说意义上的非理性）。进行投资的企业家也会受到其他企业家所作所为的影响，并且可能会出现普遍的乐观或悲观情绪。凯恩斯关于不确定性和预期后果的观点被后凯恩斯主义者采纳，并发展成现代主观预期理论。
本质上，在一个人需要在真正的不确定性下做出决定的时候，他可以 (1) 认识到这种不确定性或 (2) 陷入他们不会面临不确定性的错觉，也许是因为相信可以为未来结果提供客观概率。无论哪种方式，大多数人都有采取行动的倾向。
因此，凯恩斯可以被视为检验了企业家利用本能、惯例和思维习惯做出投资决策的heuristic and biases。但启发式不一定“保证正确的解决方案”。无论是否投资，决定都可能是错误的。决策中的认知偏差之一是从众效应，与投资者和企业家如何陷入普遍预期状态并形成集体行动有关，反之亦然。
