ideal conditions but rather with the quantitative proportions between different economic activities or sectors that are necessary in any real economy so that economic activity can continue and grow. It is concerned with all the conditions for that, including the social and technical conditions necessary for the economic process. Reproduction economics does not assume that society is kept in balance by market mechanisms. Wassily Leontief developed Marx's idea further in his input-output economics (see also input-output model). However, there is a major difference between Leontief and Marx. By treating gross profit as a "factor input" as well as a factor output, the respective total values of the input and output in Leontief's model are always exactly equal. In Marx's model, the output in an accounting period is normally always higher in value than the input. This is what Marx believed capitalists to be in business for: to produce a product sold at a higher value than the sum of input costs, thus generating profit. The profit in Marx's theory is not an "input" (it is not part of the capital advanced), but a business result, the yield of capital on an investment. In Marx's view, economic reproduction in any society has five main features: the production of reproducible products (goods or services) replacing, maintaining, or adding to the stock of society's physical assets the physical maintenance of the (working) population and their dependents through household consumption the reproduction and growth of the total population, including procreation and childrearing the reproduction, enforcement, and maintenance of social relations, in particular the relations of production that characterize the social hierarchy, and property rights the maintenance and reproduction of trading and distribution relations (the systems, institutions, and organizations enabling market trade and non-market allocation of resources) What is specific to capitalist society is that these reproduction processes are accomplished primarily via the intermediary of commercial trade; that is, they are mediated by the market. Reproduction on a larger and larger scale becomes conditional on successfully making money. It means that these processes tend to be increasingly reorganized to bring them in line with the requirements of the accumulation of capital. Marx's argument is that by producing an output value the equivalent to their own labor cost plus a surplus value (or gross profit) appropriated by capitalists, waged workers accomplish many of the processes involved at the same time. Part of the role of the