[90] We here suppose a series of productive periods already rapidly passed through, which may suffice to replace the whole of the total capital by the new technical processes. It is evident however, that as fixed capital is replaced in successive portions, in a first stage, goods are used as capital, whose cost of _reproduction_ no longer corresponds to their original cost of _production_, _i.e._ whose actual social value no longer corresponds to the original one. But to consider the separate stages would here cause a useless complication. [91] The explanation of the way in which the average rate of profit arises belongs to the fundamental part of the third book of _Das Kapital_, and Marx must have thought it out together with the fundamental chapters in the first book. [92] This is the case contemplated by Ricardo in the celebrated § 44 of chapter vi, _On Profits_: Marx appears to attach little importance to this case, having complete faith in the continued technical progress of agriculture, not to speak of other counteracting causes. It is necessary to add that Marx in conformity with his law, maintains that the rent of land also has a tendency to fall, although it may increase its total amount, or its proportion in reference to industrial profits: see vol. iii, 223-4. _CHAPTER VI._ ON THE ECONOMIC PRINCIPLE TWO LETTERS TO PROFESSOR V. PARETO I _Need for more comprehensive definition of the economic principle: Reasons why the mechanical conception erroneous, economic fact capable of appraisement: Cannot be scale of values for particular action: Economic datum a fact of human activity: Distinction and connection between pleasure and choice: Economic datum a fact of will: Knowledge a necessary presupposition of will: Distinction between technical and economic: Analogy of logic and æsthetic: Complete definition of economic datum._ _Esteemed Friend_, On reading the little paper, which you were courteous enough to send me, on how to state the problem of pure economics,[93] I at once felt a desire to discuss the subject with you. Other occupations have obliged me to defer the satisfaction of this wish until now; and this has been fortunate. The extracts from your new and still unpublished treatise on pure economics, which came out in the March number of this Review,[94] have obliged me to abandon in part the scheme of thought which I had in mind; for I saw from them that you had modified some of those points in your thesis, which seemed to me most open to dispute.