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.