Halliday & Matthiessen (1999: 509):
A biological system is a physical system with the added component of "life"; it is a living physical system. In comparable terms, a social system is a biological system with the added component of "value" (which explains the need for a synoptic approach, since value is something that is manifested in forms of structure). A semiotic system, then, is a social system with the added component of "meaning".
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Note that 'semiotic' here means 'social semiotic'. On this model, the somatic semiotic system of perception is a biological system 'with the added component of meaning'.