Light—our experience of light, our measurement of light, and the notion that light speed is constant—can be understood to mark our interface with the cosmos. David A. Grandy's book moves from the scientific to the existential, from Einstein to Merleau-Ponty, 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 predetermines 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 scientifically or philosophically inclined persons who enjoy stretching themselves in new ways.