The puzzle at the heart of this book-light speed constancy-is a bit hard to wrap your mind around, but that is part of the fun. Light-our experience of light, our measurement of light, and the notion that light speed is constant-can be understood to mark the very interface of us with the cosmos. David Grandy's book moves from the scientific to the existential, from Einstein to Merleau-Ponty and Heidegger, from light as a phenomenon to light as that which is constitutive of reality. To measure the speed of light is to measure something about the way we are measured or blended into the cosmos, and that universal blending pre-decides our measurement of light speed in favor of a universal or constant value. It's quite a trip, one aimed at scientists who have pondered light speed constancy, philosophers inclined to question the idea that mind and world are distinct, and for scientifically or philosophically inclined persons who enjoy stretching themselves in new ways.