Halliday & Matthiessen (1999: 193):
Natural forces (tides, hurricanes, etc.); instruments (as extended body parts); powered artefacts (locomotives, industrial machines, etc.): intermediate between animals and material objects. These are typically active (including effective action, moving other objects), but non-volitional; hence when the Actor is of this category, the process does not admit phases that construe intentionality (e.g. the hammer tried to force the lock).