<Tr> <Td> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> </Td> </Tr> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> <P> The three - sector theory is an economic theory which divides economies into three sectors of activity: extraction of raw materials (primary), manufacturing (secondary), and services (tertiary). It was developed by Allan Fisher, Colin Clark and Jean Fourastié . </P> <P> According to the theory, the main focus of an economy's activity shifts from the primary, through the secondary and finally to the tertiary sector . Fourastié saw the process as essentially positive, and in The Great Hope of the Twentieth Century he writes of the increase in quality of life, social security, blossoming of education and culture, higher level of qualifications, humanisation of work, and avoidance of unemployment . </P>

What are primary secondary and tertiary sectors of economy