Fawcett (2010: 21):
Halliday then points out that "the theory allows for downward 'rank shift' : the transfer of a [...] given unit to a lower rank" [i.e., it allows a unit such as a clause to occur at an element where, in the unmarked case, a lower unit such as a group or a word would occur]. Moreover the theory "does not allow for upward 'rank shift'", i.e., a word cannot function directly as an element of a clause. The claim that elements of a clause must be filled by groups rather than by words (which Halliday expresses as "no upward rank shift") has attracted particularly strong criticism, both from outside SFL (e.g., Matthews 1966) and from within it (e.g., Hudson 1971 and Fawcett 1973/81).
Blogger Comments:
This is misleading. Excluding (downward) rank-shift, structural elements of a higher rank are realised by units of the rank below — elements of a clause by groups and phrases, and elements of groups and phrases by words.
An element of clause structure, such as Subject, may be realised by a nominal group with only one structural element, Thing, realised by one word, as in the clause Cretans are liars.
The reason for distinguishing the group Cretans from the word Cretans is that each unit affords different systemic potential. For example, the group Cretans can be replaced by another group, such as the richest corporate executives, whereas the word Cretans can only be replaced by another word, such as politicians.