Not many of us are perplexed about space. We can move around in it and its nature seems experientially obvious. Yet even in the case of spatial properties, the philosophically minded can deem the existence of reliable measuring rods, capable of metricating space, as not being as straightforward a matter as commonsense might suppose. Moreover, when physical cosmologists theorise about the Universe, they find that its vast spatial domains exhibit an intrinsic curvature, corresponding to General Relativity's account of the nature of gravity. There are certainly subtleties about the nature of space, which go beyond the expectations of everyday thought, but they are nothing like as perplexing as those we encounter when we attempt to think about the nature of time.

Time travel is not available to us and we have to take our experience of time ‘as it comes to us’, in the succession of those fleeting present moments which as soon as we experience them recede immediately into the inaccessible fixity of the past. Famously, St Augustine, meditating on temporality in the Confessions, said that as long as he did not think about it, he knew what time was, but as soon as he reflected on the nature of temporal flux, he began to be perplexed. To commonsense, the one thing that does seem clear is that time flows. Yet one of the central issues in the modern discussion of temporality is whether this is indeed the case, or whether our human sense of the flow of time is merely a trick of psychological perspective, and the fundamental reality of time is quite different.