The thinker to whom I refer is Mill, who assigns to this argument a very prominent place in the opening chapter of his _Principles of Political Economy_.
Certain economists have, so he says, debated "whether nature gives more assistance to labour in one kind of industry than in another"; and he endeavours to show that the question is in its very essence unanswerable.

"When two conditions," he proceeds, "are equally necessary for producing the effect at all, it is unmeaning to say that so much is produced by one, and so much by the other.

It is like attempting to decide which of the factors five and six contributes most to the production of thirty." And if this argument is true of nature and labour, it is equally true of labour and the ability by which labour is directed.

Thus a great ocean liner which, in Mill's language, would be "the effect," could not be produced at all without the labour of several thousand labourers; and it is equally true that it could not be produced at all unless the masters of various sciences, designers, inventors, and organisers, directed the labour of the labourers in certain specific ways.

Both conditions, then, being "necessary for producing the effect at all," the portions of it due to each would, according to Mill's argument, be indeterminable.