Halliday & Matthiessen (1999: 575-6):
The most central aspect of the various changes that took place [in the move from folk to scientific models] was the reification of experience — the grammatical metaphor whereby processes were reconstrued as things. In the language of everyday commonsense, A attracts B, so B moves — a complex of two clauses; in the language of science, attraction causes (or is the cause of) movement — where the everyday clause complex, the sequence of two processes of action, has been 'compressed' into one clause with two nominalised elements, and a single process of being (cause 'be causally', or be).
When they are reconstrued as things, processes lose their location in time and often also their participants; for instance, A attracts B is likely to be reconstrued simply as attraction. Attraction, repulsion, motion, gravity, acceleration, etc. can then be taxonomised in the same way as ordinary things such as plants and animals; they become part of an explicit taxonomy of metaphorical things. These basic resources were already in place in ordinary language — the nominal group for representing things and for organising them into taxonomies, nominalising suffixes for reconstruing non-things as things, and so on; but their potential was being exploited to a greater extent and in significantly different ways.
This change in the grammar entailed a change in world view, towards a static, reified world — so much so that Bohm (1979) complains that language makes it hard to represent the kind of flux that modem physics likes to deal with. Bohm's dissatisfaction is directed at language in general; but bis real target is — or should be — the language of science. The everyday language of casual speech is, by and large, a language of flux, construing experience in much the way that Bohm seems to demand (see Halliday, 1987).