A physical system is just that: a physical system. What is systematized is matter itself, and the processes in which the system is realized are also material. But a biological system is more complex: it is both biological and physical — it is matter with the added component of life; and a social system is more complex still: it is physical, and biological, with the added component of social order, or value. … A semiotic system is still one step further in complexity: it is physical, and biological, and social —and also semiotic: what is being systematized is meaning. In evolutionary terms, it is a system of the fourth order of complexity

(M.A.K. Halliday 2005, p. 68) as cited in: Andrew Halliday and Marion Glaser (2011) "A Management Perspective on Social Ecological Systems". In: Human Ecology Review, Vol. 18, No. 1, 2011.