
(another lazy cut&paste from my Goodreads.com page)
To learn more about this great book through carefully selected and categorized quotations look at Chris Cleirigh's "sys-fun" pages. Here is a very good example.
"Language is set apart, however, as the prototypical semiotic system, on a variety of different grounds: it is the only one that evolved specifically as a semiotic system; it is the one semiotic into which all others can be “translated”; and it is the one whereby the human species as a whole, and each individual member of that species, construes experience and constructs a social order. In this last respect, all other semiotic systems are derivative: they have meaning potential only by reference to models of experience, and forms of social relationship, that have already been established in language. It is this that justifies us in taking language as the prototype of systems of meaning." Halliday & Matthiessen (1999: 509-10).
That is, we can make meaning through a variety of modes (visual, including photography, graphic design and visual art, aural, including music, and others) and as a result of a register variation (either through natural language - academic text, scientific jargon or air traffic controllers instructions - or through semiotic systems derived from natural language, particularly mathematics), but only because we can mediate their meaning through language.