Halliday & Matthiessen (1999: 555)
Synchronically (that is, viewed synoptically in this way as a meaning potential) a language is, as we have said, a probabilistic system: if we say that, in the grammar, there is a system of primary tenses past/ present/ future, we assume the rider 'with a certain probability attached to them’. But we do not, of course, speak or write with one grammatical system at a time. Systems intersect with each other simultaneously (we choose tense along with voice, polarity, mood, transitivity and so on), and they follow each other in linear succession (we choose tense in clause 1, again in clause 2, again in clause 3 and so on). Each instance has its environment, both of previous instances, and of simultaneous instances of systems with their own sets of probabilities.