Halliday & Matthiessen (1999: 539):
Why did such a significant development take place? The most important single factor was undoubtedly the evolution of science and technology. It is possible to trace the emergence of this pattern of grammatical metaphor back to the origins of western science in ancient Greece, and to follow its development step by step; each stage in the evolution of the grammar realises a stage in the evolution of a world view.
The philosopher-scientists of the ancient Greek world, Thales, Pythagoras, Anaximander and their successors, inherited a language with a grammar of the kind outlined above, in which experiential meanings were construed in clausal patterns as a balanced interplay of happenings and things; nouns enjoyed no special privileged status. In the course of their writings (and no doubt first of all in the course of their sayings, only we have no access to these) they distilled this into a language of learning. We do not know how much they reflected on this process; it is unlikely they engaged in any very explicit language planning. What they did was to exploit the resources of everyday Greek, its fundamental semogenic potential. In particular, they exploited two of its grammatical powers: the power of forming new words, and the power of extending grammatical structures.