We are concerned in this chapter with an apparently simple idea - 'if you want to develop you must industrialize'. We can think of this idea either as a theoretical proposition derived by logical reasoning from some set of assumptions or as a historical or empirical idea, a comment upon or generalization from events which have happened in the real world. However, there is a difference. For if the proposition were only historical or empirical, it would carry no force of necessity, i.e. even though we can show that almost invariably as the per capita incomes of countries have risen, so the importance of industry in their economies has increased, while the importance of agriculture has diminished, there is no logical necessity for this to be so in the future just because it has nearly always been so in the past. We cannot preclude the logical possibility that there might be ways of raising per capita incomes over the long term without industrializing. Historical or empirical evidence (which we present in Table 1) can never be absolutely conclusive proof or disproof of any theoretical proposition.