 In Prior’s opinion the notion of branching time is certainly not unproblematic. After all it is a representation of time in terms of space. The notion seems to involve the idea that the “Now” is moving through the system. Several authors have argued that the picture of a moving point within the branching time system is rather problematic. In fact, this problem of the “Now” as a moving point goes back to Jack Smart (1949). Later it has been debated by Storrs McCall (1976, p. 348, 1995) and Graham Nerlich (1995). Recently MacFarlane has pointed out that there is nothing such a motion could represent, since we have already represented time as one of the spatial dimensions of the tree (MacFarlane 2008, p. 86).