Das Kapital is the most controversial work on Political Economy ever to have been written. The subject of more and sharper controversy even than was Ricardo’s Principles, it has probably met with wider extremes of praise and denigration than any other work of its kind. The two concepts that have been the special centres of controversy have been those of property-income as surplus-value, or the fruit of exploitation, and of the historical development of capitalist society towards revolutionary transformation into socialism. Proper understanding of both these concepts depends on an appreciation of the boundaries of political economy as Karl Marx envisaged them. For him the ‘social relations of production’ were included as well as the ‘productive forces’ and the conditions of exchange. Qualitative characterisation of relationships was as important as was a solution of the quantitative problem of value and of the derivation of prices from values.