Aristotle long ago posed a puzzling question about time. Does it belong to the class of things which exist, or to the class of things which do not exist? In Book IV (chapter 9 ) of the Physics he supplies us with reasons for thinking it belongs to the realm of the non-existent. After all, "one part of it has been and is not, while the other is going to be, but is not yet . . . One would naturally suppose that what is made up of things which do not exist could have no share of reality." In addition to the past and future there is, of course, the present. But if, as it can seem natural to suppose, this is no more than the durationless boundary (or limit) separating the past and future, then even if we hold the present to be among the things which exist, we have not succeeded in gaining much by way of reality for time per se .This puzzle is deep, and philosophical opinion remains divided as to how best to respond to it, with some holding that reality is confi ned to the present, others holding that the past and/or the future also belong to the realm of the real. Rather less momentously, Aristotle ' s puzzle also arises in connection with our experience of temporality. Aristotle suggested that there is an intimate relationship between time and our awareness of change. Irrespective of whether he is right about that, he was surely right in thinking that change is something which fi gures in our ordinary experience. On listening to the regular beat of a drum (or the slow throb of a heart-beat, or the pulse of light on a sleeping computer), most of us can tell if the frequency increases, decreases, or remains unchanged. In so doing, we are displaying an ability to compare the temporal intervals between perceivable events, and estimate their magnitudes. But although this ability is a useful one, and much studied by psychologists interested in "time perception," we are able to apprehend change in a more vivid and direct manner. 1 When we look through the window of a speeding train, we see the countryside fl owing smoothly by. Holding one ' s hand under a running tap produces a tactile analogue: the fl owing 23 A Companion to the Philosophy of Time, First Edition. Edited by Heather Dyke and Adrian Bardon.