Halliday & Matthiessen (1999: 510):
… we do not yet fully understand the nature of the relationship that is the semiotic analogue of the "cause : effect" of classical physics: this is the problem of realisation. It is true that Saussure, and even more Hjelmslev, took important strides towards an understanding; but we are still arguing about what Saussure really meant (to us it seems that he had not clearly separated the two concepts of instantiation and realisation), and Hjelmslev has been largely ignored — Sydney Lamb (eg 1966a, b) is almost the only person who has tried to follow through his achievements.
