onal lines:
In order that equilibrium should be stable, it is necessary that a slight movement away from the equilibrium position should set up forces tending to restore equilibrium....
but concluded that stability requires that
. . . a rise in price makes supply greater than demand, a fall in price demand greater than supply.
This is indeed the stability condition corresponding to a dynamic system in which excess demand causes a rise in price, but, as Samuelson pointed out, the proposition is not explicitly derived as the condition of convergence of such a dynamical system. The problem may be considered trivial in the case of a single market,3 but it raises difficulties in analysis and interpretation in the case of multiple exchange (exchange of more than two commodities), as Hicks foresaw (, p. 66):
What do we mean by stability in multiple exchange ? Clearly, as before, that a fall in the price of X in terms of the standard commodity will make the demand for X greater than the supply. But are we to suppose that it must have this effect (a) when the prices of other commodities are given, or (b) when other prices are adjusted so as to preserve equilibrium in the other markets?
To resolve the difficulty Hicks introduced his concepts of perfect and imperfect stability. He noted first that it was necessary to distinguish a series of conditions-that a rise in the price of X will make supply greater than demand (1) all other prices being given, (2) allowing for the price of Y being adjusted to maintain equilibrium in the Y market, (3) allowing for the prices of Y and Z being adjusted, and so on, until all prices have been adjusted. He then defined as imperfectly stable a system in which a rise in price of a commodity causes excess supply for the commodity after all repercussions are allowed for, and as perfectly stable a system in which a rise in price causes excess supply regardless of how many other prices are adjusted to attain equilibrium values in their respective markets.4
Samuelson  criticized Hicks' concept of