ABSTRACT
In the history of the MPTD, the expression ‘marginal product of labour’ (MPL) and the preposition ‘of’ have been used in two different senses. For J.B. Clark, MPL meant the portion (of the joint product) that is specifically produced by labour. The ‘of’ is given proprietorial significance, and allows normative conclusions to be drawn. When other writers used a non-normative version of the MPTD, and defined MPL as the change in total product that occurs after a change in the amount of labour employed, they were using ‘of’ in a sense quite different from J.B. Clark’s. Their ‘of’ has no proprietorial significance, and does not lead to Lockean-based normative conclusions. If they were aware of their different meaning of ‘of’, they rarely drew their readers’ attention to it.
