Halliday & Matthiessen (1999: 184):
The theoretical principle that a participant can be defined as the potential carrier of an attribute holds for participants of all kinds. We need however to make one proviso: namely, that more or less any figure can be construed metaphorically "as if" it was a participant. This is a central feature of grammatical metaphor. An example would be Our earlier encounter with this species [had led us to believe that...] where 'encounter' is semantically a process: cf. We had encountered this species earlier [and as a consequence we believed that...]. If 'encounter' is construed congruently in the grammar as a process, it cannot enter into an ascriptive figure. But once it is metaphorised into a participant, it can: our earlier encounter with this species had been almost disastrous. This in fact is one of the discursive contexts favouring this type of grammatical metaphor, and hence serves indirectly as a further illustration of the general principle we have outlined.