Halliday & Matthiessen (1999: 612):
These already foreshadow the semantic motifs of the adult language, the experiential and interpersonal metafunctions, although they are not in any direct correspondence with them; thus the "personal" signs expressing curiosity, or pleasure/ displeasure, constitute the beginning of the semiotic exploration of experience and open the way to naming and classifying phenomena, while the interactional signs are the ones whereby a child enacts social relationships with caregivers and others who are close (Halliday, 1975; 1984b). Here we see the earliest context for the later emergence of types of process within the grammar (Halliday, 1991).