Note also that even a high value modal (‘certainly’, ‘always’) is less determinate than a polar form: that’s certainly John is less certain than that’s John; it always rains in summer is less invariable than it rains in summer. In other words, you only say you are certain when you are not.
Wednesday, 27 September 2017
High Value Modality Vs Polarity
Halliday & Matthiessen (2014: 177):