I would argue, for the nothing it is worth, that there is a natural rate of interest. It is the percentage growth in productivity. It is the increased value money would attain due to increases in productivity if personal income were uniform and fixed, such that not spending it would result in gained 'purchasing power'. Since incomes are never either, such a natural form of interest cannot be realized. Still, any rate of interest that is more than the rate of growth in productivity is usury. As a tool for 'managing' the economy interest rates are, as pointed out in this essay, ideology (in the service of the 'investor class') in the guise of economics.