Typical causal theories of time say that the order of time is determined by the order of causal relationships between events in time. This tends to be a difficult theory to develop, if only because of the possibilities of simultaneous causation and time travel. But suppose with substantivalists that there really are moments (or intervals) of time. Then it is possible to have a very simple and elegant theory of the order of time:
- Time u is prior to time v if and only if time u causes time v.
There is also an interesting and slightly more complex variant:
- Time u is prior to time v if and only if some event at u at least partially causes v.