Systemic functional linguistics (SFL) is an approach to linguistics that considers language as a social semiotic system. It was developed by Michael Halliday, who took the notion of system from his teacher, J. R. Firth. Whereas Firth considered systems to refer to possibilities subordinated to structure, Halliday in a certain sense "liberated" the dimension of choice from structure and made it the central organising dimension of this theory.