Halliday & Matthiessen (1999: 611-2):
A human infant is a social being from birth (cf. Trevarthen, 1987). Newborn children can exchange attention with their mothers, addressing them and recognising that they are being addressed by them; the infant's whole body is actively involved in the exchange. This is "pre-language" ("pre-meaning", even "pre-text"); but it is not language — no distinction is yet being made between symbolic and non-symbolic acts.
Then, as they become aware of themselves and their environment, children feel a tension building up between two facets of their experience: between what they perceive as happening "out there" and what is happening "in here", within their own borders so to speak. We can watch babies of around 3 - 4 months struggling to reconcile these complex sensations: they can see a coloured object, reach out, and grasp it and pull it towards them. The inner and the outer forms of this experience have to be brought into line; in order to achieve this, children begin to act in a new, distinctively symbolic mode.
A typical example of such an "act of meaning" is the high-pitched squeak a child of around 5 months may produce when some commotion takes place that has to be assimilated. Adults interpret these proto-signs as a demand for explanation: "Yes, that's a bus starting up. Isn't it noisy!" Thus meaning arises out of the impact between the material and the conscious as the two facets of a child's ongoing experience.