Comment on Lars Syll on 'IS-LM is bad economics no matter what Krugman says'
Blog-Reference
There seems to be consensus that it is alone IS-LM that concerns us here.
The main point is rather simple: the General Theory (GT) is a loose composition of formal and verbal arguments. The problem is that most of Keynes's verbal arguments cannot be derived from the foundational formalism. This is not to say that the verbal arguments are factually wrong, it means that, in the best case, they are hanging in the air. This does not diminish the value of the GT as a piece of political opinion making but certainly as a piece of theoretical economics.
In the GT the original formalism and the original interpretation (oK) do not match. In addition, the formalism is indefensible with regard to the treatment of income and profit. With IS-LM Hicks provided a second interpretation (H). The (oK)-interpretation and the (H)-interpretation do not match either. What is more, we have a new interpretation but the foundational formalism is still defective. That is, the (H)-interpretation is also hanging in the air.
Krugman has been reprimanded for not admitting that interpretations (oK) and (H) are incompatible. The salient point is, instead, that Krugman and Hicks before him have not realized that, to begin with, Keynes's formal foundations are indefensible with regard to the treatment of income and profit. This is the oversight that counts from the viewpoint of theory building.
In sum: Keynes, Hicks, and Krugman provide different interpretations of the same flawed formalism. In order to advance economics from a talk-show to a science, all three interpretations have to be rejected on purely formal grounds.