Recently, I had a dream where I was trying to explain supply and demand to an audience of intransigent economists.How could I say that demand curves sloped downward, they asked, when for so many goods, a simple plot of quantity demanded against price showed the opposite? How could these curves be useful concepts when, even by the most generous account, their parameters shifted from year to year? I knew the answers. The incorrect slope of demand was no surprise; a plot of quantities against prices would reveal the demand curve only if the variation was caused by supply shocks. And yes, supply and demand didmove around from year to year, but this did not invalidate the concepts. If I am selling oil, I can predict that a sudden decline in supply (embargo) will increase the price, I can predict that a new source of supply (fracking) will decrease the price, and I can predict that a decline in demand (fuel efficiency) will decrease the price. All of this is useful information. The dream audience was unmoved. But upon waking up, I was relieved to find myself in a world where economists are not so silly. We understand that supply and demand is a useful framework, even if there is an identification problem and even if there is no consensus on the parameters for a particular market. In fact, we persist even in the face of more profound theoretical complications, like imperfect competition or increasing returns. Then I started reading commentary on the Phillips curve and economists started seeming awfully silly again. Somehow, a weak reducedform relationship in the aggregate data has led many people to deny the Phillips curve as a structural relationship. In the face of this criticism, McLeay and Tenreyro’s paper is a vitally important rejoinder.

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最近，我做了一个梦，我试图向一群顽固的经济学家解释供求关系。我怎么能说需求曲线向下倾斜，他们问，对于这么多商品，什么时候可以用简单的需求量与价格的关系图显示相反？这些曲线如何成为有用的概念，即使是最慷慨的说法，它们的参数每年都在变化？我知道答案。需求的不正确斜率不足为奇。只有当变化是由供应冲击引起时，数量与价格的关系图才能揭示需求曲线。是的，供需确实每年都在变化，但这并没有使这些概念无效。如果我卖石油，我可以预测供应突然下降（禁运）会提高价格，我可以预测新的供应来源（水力压裂）会降低价格，我可以预测需求下降（燃料效率）会降低价格。所有这些都是有用的信息。梦中的观众不为所动。但醒来后，发现自己身处一个经济学家并不那么愚蠢的世界，我松了口气。我们理解供需是一个有用的框架，即使存在识别问题，即使对特定市场的参数没有达成共识。事实上，即使面对更深刻的理论复杂性，如不完全竞争或收益递增，我们仍然坚持。然后我开始阅读关于菲利普斯曲线的评论，经济学家们又开始显得非常愚蠢。不知何故，汇总数据中的弱简化关系导致许多人否认菲利普斯曲线是结构性关系。面对这种批评，McLeay 和 Tenreyro 的论文是一个至关重要的反驳。