Factor complementarity and substitution are phenomena belonging to different provinces of the realm of action. Complementarity is a property of means employed for the same end, or a group of consistent ends. All the means jointly employed for the same end, or such ends, are necessarily complements. Factor complementarity presupposes a
plan within the framework of which each factor has a function. . . . Substitution, on the other hand, is a phenomenon of change the need for which arises whenever something has gone wrong with a prior plan. Substitutability indicates the ease with which a factor can be turned into an element of an existing plan. . . . The importance of substitutability lies in that it is usually possible to pursue the same end (output) with a different combination of factors. The importance of complementarity lies in that “technical rigidity” (invariability of the mode of complementarity) may often make it necessary to change the end rather than the means; an existing combination of factors is used to produce a different output.
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