John Maynard Keynes' famous saying, "In the long run we are all dead", is from Chapter III of A Tract on Monetary Reform. He describes, in Chapter II of this 1924 book, how governments can obtain resources from their citizens through a deliberate policy of inflation. In this sense, inflation is like taxation. He also discusses how people might react to such a policy, making it difficult for the government to "tax" at the same rate without constantly raising the rate of inflation.
In Chapter III, Keynes states a general principle:
"...a large change in [the quantity of cash], which rubs away the initial friction, and especially a change in [the quantity of cash] due to causes which set up a general expectation of a further change in the same direction, may produce a more than proportionate effect on the [price level]. After the general analysis of Chapter I. and the narratives of catastrophic inflations given in Chapter II., it is scarcely necessary to illustrate this further, - it is a matter more readily understood than it was ten years ago. A large change in [the price level] greatly affects individual fortunes. Hence a change after it has occurred, or sooner in so far as it is anticipated, may greatly affect the monetary habits of the public in their attempt to protect themselves from a similar loss in future, or to make gains and avoid loss during the passage from the equilibrium corresponding to the old value of [the quantity of cash] to the equilibrium corresponding to its new value. Thus after, during, and (so far as the change is anticipated) before a change in the value of [the quantity of money], there will be some reactions on the values of [the parameters of the quantity equation in Keynes' Cambridge formulation], with the result that changes in the value of [the price level], at least temporarily and perhaps permanently (since habits and practices, once changed with not revert to exactly their old shape), will not be precisely in proportion to the change in [the quantity of cash]." -- J. M. Keynes, pp. 81-82.
It seems to me that the above is the Lucas critique, but with a more realistic understanding of human behaviour. What exactly did Lucas contribute again?