Halliday (2008: 35):
The grammar of every language is (in one of its metafunctions, the ideational) a construal of human experience: it constructs our “reality” by transforming our experiences into meanings. And in doing this, the grammar often has to choose: to choose either one way of seeing things, or the other. For example, think of time. Either time is a linear progression, out of future through present into past; or else it is a translation from the virtual into the actual. It can’t be both. We may choose to model it (and note here that I am talking about our grammar — not our theory of grammar, our “grammatics”; so we means the speakers of the language, not the linguists) … so let us say our language may choose to model it either as tense, or as aspect;