"Days, months, and years were given to us by nature, but we invented the week for ourselves. There is nothing inevitable about a seven-day cycle, or about any other kind of week; it represents an arbitrary rhythm imposed on our activities, unrelated to anything in the natural order. But where the week exists—and there have been many cultures where it doesn't—it is so deeply embedded in our experience that we hardly ever question its rightness, or think of it as an artificial convention; for most of us it is a matter of 'second nature.'