Halliday & Matthiessen (1999: 416):
Many traditional notions of meaning are of the second kind — meaning as reference, meaning as idea or concept, meaning as image. These notions have in common that they are 'external' conceptions of meaning; instead of accounting for meaning in terms of a stratum within language, they interpret it in terms of some system outside of language, either the 'real world' or another semiotic system such as that of imagery.