Halliday & Matthiessen (1999: 156):
Semantically, the nucleus construes the centre of gravity of a figure, the focal point around which the system of figures is organised. When we describe the Medium as “actualising” the Process, we are really saying that the unfolding is constituted by the fusion of the two together — there can be no Process without an element through which this process is translated from the virtual to the actual.