Halliday & Matthiessen (1999: 96-7):
… young children will typically construe concrete phenomena that are part of the field of visual perception they share with their interactants. In other words, they are construing into linguistic meanings their experience of the material world as it is construed in the categories of another semiotic system, viz. (visual) perception. These extra-linguistic categories are construed as the signification of the semantic categories of the ideation base — always in some particular situation when the child first engages with them. To construe experience of concrete phenomena as meaning is thus to construe some signification which lies outside the ideation base as a value which is internal to the ideation base system. Part of the power of categorisation is that extra-linguistic phenomena that are quite varied in signification can be construed as alike in value.