Halliday & Matthiessen (1999: 585, 586):
The grammar thus construes sensing as a bounded domain within our total experience of change. This picture is further enriched through lexis, prominently through lexical metaphors. Metaphors relating to space, with the mind as a container, a finite space or a physical entity reinforce the grammar’s construal of a bounded domain of sensing. …
This mind-space may enter into material processes of storing, searching, crossing, escaping etc, either as participant or as circumstance, and also into relational processes of “being + Location”. It is interesting to note that in these various lexical metaphors the Sensers are still very much present; they are not effaced. In fact, a number of these lexical metaphors constructed on the model of material clauses retain the option of projecting … [eg] he kept in mind that the moon was a balloon …