The accounting identities equating aggregate expenditures to production and of both to incomes at market prices are inescapable, no matter which variety of Keynesian or classical economics you espouse. I tell students that respect for identities is the first piece of wisdom that distinguishes economists from others who expiate on economics. The second? … Identities say nothing about causation.
– James Tobin, 1997, p. 300, ‘Comment’, in B.D. Bernheim and J.B. Shoven (eds), National Saving and Economic Performance, Chicago: University of Chicago Press.
This is such a nice quote by James Tobin. Almost all economists, orthodox or heterodox would agree with it I believe.
In practice, however, economists confuse identities for behaviour and causation no end. They even confuse identities themselves. But it now seems that some think that usage of national accounting identities produces erroneous conclusions.
In a series of posts, (here and few posts before), David Glasner, the author of the blog Uneasy Money — Commentary on monetary policy in the spirit of R. G. Hawtrey, seems to be suggesting that letting identities go is the way forward for macroeconomic modeling.
There are two reasons why defining savings and investment to be identically equal in all states of the world is not useful in a macroeconomic theory of income. First, if we define savings and investment (or income and expenditure) to be identically equal, we can’t solve, either algebraically or graphically, the system of equations describing the model for a unique equilibrium.
[boldening and emphasis added]
So it seems that using accounting identities in your model would lead to inconsistencies. I and a few other commenters have tried to convince Glasner of his errors in series of posts.
Some people seem to think that identities do not tell anything. The truth is not so straightforward. Identities constrain outcomes. Any macroeconomic model which does not use identities as constraints may produce non-possible states of the world.
Brad deLong confronted Glasner on Twitter with this point:
— J. Bradford DeLong (@delong) February 20, 2015
If you have time, interest and energy, please convince Glasner that accounting identities cause no issues in macroeconomic modeling.