Halliday & Matthiessen (1999: 72):
From a systemic-functional point of view, the intellectual context was rather different. Systemic-functional theory was not oriented towards philosophy and logic, and the Aristotelian conception of a category did not figure as a traditional frame of reference that therefore had to be rejected. Already at the outset of the theoretical work that was to become systemic-functional theory, a major descriptive focus was on intonation (see e.g. Halliday, 1963a, b, 1967), a domain that clearly cannot be construed in Aristotelian terms; and the notion of cline was part of the theory already in the first major statement (see Halliday, 1961). Further, the system was conceived of as a probabilistic one even in proto-systemic work (see e.g. Halliday, 1956).
Systemic-functional work on 'categorisation' has thus not engaged with the philosophical tradition; nor has it tended to proceed by experimental methods. Rather, it has been concerned with how meaning is construed in naturally occurring text, e.g. in child language studies (Halliday, 1975 onwards; Painter, 1996) and in factual writing (e.g. Halliday & Martin, 1993; Harvey, in prep.). Here one of the questions has been what resources are available in the semantic system for construing new meanings — for 'category development', both in the ontogenetic time frame and in the logogenetic time frame.