ABSTRACT
A major theme in this attempt to trace and interpret the volatile history of the MPTD is the distinction between two possible meanings of the expression ‘the marginal product’ of a factor of production. On the one hand, the expression can mean the portion of the product that has been actually produced by the marginal unit of the factor. On the other hand, it can mean the change (in the total output) that occurs after the addition or subtraction of the marginal unit of the factor. Both meanings have been used, and are still being used, in the MPTD literature, often without being clearly distinguished. Given the centrality of the distinction in the following study of the history of the MPTD, it is essential that it be explained and justified in some detail. To simplify the explanation the following definitions and acronyms are deployed:
• SMPL, or the specific marginal product of labour. When labour (the variable factor) is combined with other (fixed) factors in a productive process, the SMPL is the portion of the product that has been physically produced or caused by the marginal unit of labour, as distinct from the portions causally attributable either to the fixed factors or to previous, intra-marginal units of labour. Similarly, SMPK and SMPN are the specific marginal products of capital and of land (or natural resources).
