"The Celts believe that the visible and invisible worlds, the material and the spiritual, were one. For them, certain places were sacred-places where the divide between visible and invisible was very thin, where the presence of the spiritual was almost palpable. They revered such "thin places" as "sacred space." In this book, Margaret Silf introduces seven traditional sacred spaces: the infinite knot, the Celtic cross, hilltops, wells, groves and springs, thresholds and crossing places, and boundaries."