Quaternary sector" is a further delineation of the three-sector hypothesis of industry in the sense that the quaternary sector refers to a part of the third or tertiary sector along with the quinary economic sector. It has been argued[by whom?] that intellectual services is distinct enough to warrant a separate sector and not be considered merely as a part of the tertiary sector.[citation needed] This sector evolves in well-developed countries and requires a highly educated workforce.[3]