1 The actual economy is in a constant state of change; economic theory also changes and develops. This chapter will investigate whether there is some kind of congruity between these phenomena. Obviously, the economy is the basis of people's lives, while economic theory or one's view of the economy belongs to the sphere of ideology. There are two distinct approaches to this congruence problem. Marxists regard ideology as a reflection of underlying basic material conditions, while Max Weber emphasizes the importance of the reverse relationship, from ideology to economy. In our particular case, however, I consider that Weberian relationships are not significant, though I am ready, on the other hand, to accept that it is easy to point out a number of instances where actual economies have been influenced by economic theory. No one can deny that in many western economies the influence of Keynesian doctrines was apparent at least for some time after the War. The Bretton Woods regime of fixed exchange rates which worked from 1950 to 1970 is a well-known example. I shall later point out an economic structural change which was created by Ricardo's economics.