Halliday & Matthiessen (1999: 213-4):
… grammar’s model of time has been evolving unconsciously in the context of human survival; it is part of the selective and collective wisdom that the species has accumulated in the understanding of its relationship to its environment and in the interaction of its members one with another. And again, like everything else in the construal of experience it is the product of continual compromise, whereby divergent and often conflicting aspects of experience are adjusted and accommodated in such a way that all of them have some place in the total picture.