The familiar words "space," "time," "knowledge," taken in this order, tend to slip away in their meaning. In common sense terms, "space" is identified with a continuous extent, seen either as a void or as something that contains things within it, be it a speck of dust, a cluster of galaxies, or even the universe as a whole. "Time" is vaguely interpreted as a transition from the past to the future or as an environment in which events are directed in such a way that we talk about the flow or flow of time or about our awareness, carried in time and through time.