History of economics lesson.
Keynes is discussing how to use the quantity theory of money as an analytical tool.
What he is saying is that you cannot assume that you can analyze the consequences of an altered time path of the quantity of cash in the economy--n, in Keynes's notation--without considering whether the public's demand for real cash balances k, the public's demand for real checking-account balances k', and banks' desired reserves-to-deposits ratio r will also change. This is a principle that today's economists call the "Lucas Critique". (No, it is not clear to me why they do not call it the "Keynes Critique".) And this critique is correct: assume that those three other variables are not themselves altered when you consider an altered path for the money stock is, as Keynes says in the sentence after "in the long run…", for economists to set themselves too easy a task--it sweeps all the problems of analysis under the rug--and too useless a task--it generates predictions that are simply wrong.
In this extended discussion of how to use the quantity theory of money, the sentence "In the long run we are all dead" performs an important rhetorical role. It wakes up the reader, and gets him or her to reset an attention that may well be flagging. But it has nothing to do with attitudes toward the future, or with rates of time discount, or with a heedless pursuit of present pleasure.
So why do people think it does?
Note that we are speaking not just of Ferguson here, but of Mankiw and Hayek and Schumpeter and Himmelfarb and Peter Drucker and McCraw and even Heilbronner--along with many others.….Grasping Reality with the Invisible Hand
Why Did Keynes Write "In the Long Run We Are All Dead"?
Brad DeLong | Professor of Economics, UCAL Berkeley