Halliday (2008: 185):
[This complementarity] is in fact the very beginning of semiosis — of meaning as a distinct mode of behaviour, distinct from material action. At the very beginning of life, each act (crying, smiling, waving limbs about) is recreated on each occasion; it is not dependent on memory of previous acts. There are instances, but there is no underlying system — no persisting resource such that each act comes as its realisation. Then the time comes when the child retains the act in the form of memory; it is not reinvented on each occasion, but as it were cloned from stock. […] When this moment has been reached [by the age of 5 months] the child has taken the first step towards his first semiotic system, the “protolanguage” …