One possibility is that ordinary existence involves one kind of time whereas time travel, including backwards time travel, involves another kind of time. Lewis distinguishes between personal time and non-personal time:
I [distinguish] time itself, external time as I shall also call it, from the personal time of a particular time traveller: roughly, that which is measured by his w'ristwatch. His journey takes an hour of his personal time, let us say; his w'ristwatch reads an hour later at arrival than at departure. But the arrival is more than an hour after the departure in external time, if he travels toward the future; or the arrival is before the departure in external time (or less than an hour after), if he travels toward the past.
(Lewis 1976/1991, 136)
Presumably, for Lewis, “time itself” is time that we all ordinarily exist in. Thus, we have:
- • Persona/ time: the temporal order and durations of events according to an individual.
- • External time: the temporal order and durations of events according to everything else. As the opposite of personal time, call this public time.*
For example, if I time travel to Ancient Greece, there is a moment when I stand in my room in 2020 CE and a later moment when I stand in 300 BCE. In this description, 2020 CE is earlier than 300 BCE. The “later” and “earlier” are terms for events in my personal time. Yet, for everyone else, 2020 is later than 300 BCE. This “later” is a term for events in public time. Furthermore, I travel backwards in time because my personal time is the reverse of public time.
One difficulty with this distinction as a means of determining backwards time travel is that it depends on a prior understanding of public time. We can assume everyone has personal time: it is just what the temporal properties and relations are according to each person. These include the duration and temporal order of events according to each person. When a person occupies time, as far as I know, as anyone reading this occupies time, their personal time is the same as public time. We all agree on the order and duration of events.
It is the difference to public time that makes one a time traveller. Yet, what makes public time not merely another personal time, for example, a group personal time?
Perhaps there is nothing to distinguish them. Public time is only the order and duration of events which most people agree with, and which the least people disagree with. In that case, it is possible for a few people to agree on events while disagreeing with public time. You and I might both travel backwards in time, such that the order of events for both of us is the reverse of public time. But, say, if enough of us travelled in the same direction as us, we would be in public time, as would anyone else sharing our personal time. The difference is a matter of numbers.
We need not worry about this distinction, however. As time travel is typically conceived, this is not how time travel works. The order of public time is overwhelmingly shared by the greater number of individuals. In it, the dinosaurs existed earlier than humans, and the sun’s death happens later than humans. That something’s temporal order differs from that order is enough for it to be travelling backwards in time.