 This is intuitively powerful, since it’s natural to think of our future as being “open” in the branching way suggested by the road analogy and to associate this kind of branching structure with freedom of choice. But several crucial assumptions have been smuggled into this picture: assumptions about time and causation and assumptions about possibility. The assumptions about time and causation needed to make the analogy work include the following: that we “move” through time in something like the way that we move down a road, that our “movement” is necessarily in one direction only, from past to future, that the past is necessarily “fixed” or beyond our control in some way that the future is not. These assumptions are all controversial; on some theories of time and causation (the four-dimensionalist theory of time, a theory of causation that permits time travel and backwards causation), they are all false (D. Lewis 1976; Horwich 1987; Sider 2001; Hoefer 2003).