Halliday & Matthiessen (1999: 505-6):
The system often embodies complementarities providing more than one way of construing similar types of experience — for example, construing perception either as inert sensing or as active behaviour, or construing phenomena of all kinds either congruently or metaphorically. If one is concerned with a restricted, designed semiotic system, then the appropriate strategy, when one is faced with more than one option in natural language, may be to select one canonical form; but if one’s concern is with language as a whole, the only response that will hold up in the long term is to acknowledge the variation and explore its function.