For most of human history, measurement stayed close to the body. A hand was a unit of length, four inches, used to measure the height of a horse. The thumb across its knuckle gave us an inch. A foot was a foot. These were not arbitrary conventions so much as traces of older ways of knowing, using the body as our first reference point for making sense of what surrounds us. In this sense, the body was our first tool. As tool-making evolved, the body remained the starting point.