The Textual Motivation Of Metaphorical Mental Clauses Of Perception

Halliday & Matthiessen (1999: 401-2):

Metaphorical mental clauses of perception, such as the fifth day saw them at the summit (cf. Halliday, 1985: 324-5), may serve to create two quanta of information, Senser (Theme) + Phenomenon (New); consider the following example:

Summer finds campers and hikers descending on the mountains in throngs

Here the metaphorical clause summer finds campers and hikers descending on the mountains in throngs is textually motivated in terms of both THEME and INFORMATION. From the point of view of THEME, it provides summer as the unmarked Theme of the clause — one instalment in the method of development. From the point of view of INFORMATION, it groups campers and hikers descending on the mountains in throngs as one quantum of information (the Phenomenon of the metaphorical mental clause); contrast the congruent version, which has the same Theme (marked) but a different culmination of the New: 

In summer, campers and hikers descend on the mountains in throngs.