Many thanks for your letter – it is a valuable addition to my museum and I shall hang it next to an extract from Sidgwick where, after lecturing Ricardo on a quantity of labour, he goes on cheerfully himself to talk of quantities of utility.
If one measures labour and land by heads or acres the result has a definite meaning, subject to a margin of error: the margin is wide, but it is a question of degree. On the other hand if you measure capital in tons the result is purely and simply nonsense. How many tons is, e.g., a railway tunnel?
If you are not convinced, try it on someone who has not been entirely debauched by economics. Tell your gardener that a farmer has 200 acres or employs 10 men – will he not have a pretty accurate idea of the quantities of land & labour? Now tell him that he employs 500 tons of capital, & he will think you are dotty – (not more so, however, than Sidgwick or Marshall).
— Letter from Piero Sraffa to Joan Robinson, quoted in Harcourt, G.C. (1997). A “Second Edition” of the General Theory, vol. 1. New York: Routledge, p. 131.