Pasinetti On "Non-Substitution" Theorem

"...in a production context...it makes no sense to talk of 'endowments' of given physical quantities if these physical quantities, to be carried over from one period to another, are the unknowns to be determined. It makes no sense to talk of 'scarce' resources, if these resources can be produced in whatever quantities may be needed by the economic system...

When all inputs are themselves produced, a change in the composition of demand simply means that more of some inputs and less of other inputs will have to be produced, while the optimum technique remains the same. In other words, the process of adaptation to any given change in the composition of final demand is, in a production context, radically different from the one considered by traditional theory. Whereas, with given and fixed inputs (the traditional case), the only way to adapt is through a change of technique which may allow the substitution of some inputs for others, in a production context in which all inputs are themselves produced the obvious way to adapt is to produce the inputs which are needed and to cut down production of those which are no longer needed. There is no question of changing the technique. Input substitution, in a production context, has no role to play...

Another route which has been pursued to minimize the importance of the new results...consists in attributing the irrelevance of substitution to the 'very special' case of no joint production and constant coefficients [ = constant returns to scale -RLV ]. But the inconsistency of this contention is here brought into sharp relief by the very analysis of the previous pages...

As already pointed out...the joint production and nonconstant coefficients case is more complicated than, but not basically different from, the case concerning single products and constant coefficients. The complication arises from the fact that a change of the composition of demand may entail a change of the optimum technique and of the price structure. However, this does not enable us to say anything about the direction in which the input proportions will change.

...It is precisely the unambiguous direction in which relative prices and input proportions are related to each other that justifies talking of 'substitution.' But there is nothing of the sort in a production context. No general relation exists between the changes in the price structure and changes in the input proportions. More specifically, no monotonic inverse relation exists, in general, between the variation of any price, relative to another price, and the variation of the proportions among the two inputs to which these two prices refer. When this is so, to talk of 'substitution' among these inputs no longer makes any sense." -- Luigi L. Pasinetti, Lectures on the Theory of Production, Columbia University Press, 1977, pp. 186-188