The Close Bonding Of Medium And Process: Restrictions On Medium

Halliday & Matthiessen (1999: 155):

Of all the participants, the Medium is the most restricted in terms of the range of phenomena that may function in that rôle. We can see this in relation to the general types of figure: 

type of figure	range of phenomena functioning as Medium
doing	phenomenon (of any kind — but not a metaphenomenon, i.e. not fact)
sensing	conscious being
saying	symbol source
being	phenomenon (of any kind, including metaphenomenon)

[…] In other words, whatever the type of figure, the participant that is most closely bonded with the Process is the one that takes on the generalised rôle of Medium; it is this that is in a relation of mutual expectancy with the Process.