Halliday (2008: 38-9):
I have always held the view that language is a probabilistic system. Word frequency is the manifestation of probability in the lexicogrammar — or rather, of one particular aspect of it. Another, more powerful aspect of lexicogrammatical probability is the relative probability of the terms in a grammatical system. A grammatical system, such as polarity, or mood, or tense, is a small set of contrasting options with a defined condition of entry — but it is more than that: it is a set of options each having a certain measure of probability.