Growth theory offers two plausible explanations of growth. One stresses the supply of productive ideas and holds that the industrial revolution had to wait until we had thought up enough inventions to lift us into the era of modern growth. It says, roughly, that the growth of living standards depends on the growth of science. The other explanation stresses incentives: Growth could begin only when hard work and business enterprise were free of heavy taxation, of social stigma and of other interference by the government and the church. The first branch of theory is well developed; it is the second that now challenges the growth economist to explain not just growth, but the evolution of political and religious institutions and social attitudes as well.