Time is one of the richer and more enduring of philosophical topics because it is one of the more enduring of puzzles. “What, then, is time?” asked Augustine of Hippo, awe and uncertainty in his voice. “If no one asks me, I know what it is. If I wish to explain it to him who asks me, I do not know.”
He might well have added to this plaintive cry that if someone else were to offer to tell him what it is, the explanation would prove equally unhelpful. For example, the American Heritage Dictionary gives us this definition: “A nonspatial continuum in which events occur in apparently irreversible succession from the past through the present to the future.” Clear as mud! And that is one of the more straightforward definitions one can find. The statements of present-day philosophers—when they can be persuaded to provide definitions at all—are likely to be hopelessly impenetrable to the layperson.
To be fair, though, the difficulty is not the fault of the philosophers. The difficulty lies in the object itself. Augustine was right about that; there is something vexingly and permanently elusive about time. How is it that we all can know what time is, but cannot venture anything definite about it? Is time something substantial, that has an existence entirely apart from us? Or is it, as Kant argued, merely one of the preexisting cognitive vessels into which the stuff of human experience is poured? Who can say?