Halliday & Matthiessen (1999: 179):
Both types of element begin with a location in the ‘here & now’: they construct a path from the spatio-temporal moment defined instantially by the interaction base — the ‘here & now’ of the act of speaking — to a primary category of ideational phenomena. In other words, both types of group include deixis. But the deixis is of two different kinds: nominal deixis (such as near/remote) and verbal deixis (such as past/present/future), structurally realised as Deictic and Finite respectively.