Mundell  demonstrated that in order to achieve balance of payments equilibrium and full employment, monetary policy should be paired with external balance and fiscal policy with Internal balance. The fundamental problem posed by Mundell concerned the methods governmental authorities should utilize to insure that both internal and external balance would be achieved when the underlying structural parameters of the economy were unknown. Mundell argued that the assignment of monetary policy to external balance and fiscal policy to Internal balance followed from the Principle of Effective Market Classification [20, p.76j: "Policies should be paired with objectives on which they have the most direct effect."
Enders, Walter and Lapan, Harvey E., "The Assignment Problem and the Speed of Adjustment" (1979). Economic Staff Paper Series. 100.