For architects, space is the substance of design. The sciences studied space extensively too, but for their own purposes, which didn’t lead them to address the architects’ question: what space is for a person moving through a built environment. When architects tried to articulate their own understanding, they sometimes reached for scientific terms, such as 'field' and 'force,’ because those terms pointed toward something they recognized in their experience. Meantime, two traditions within science were arriving, independently, at the idea that experienced space is not a system of coordinates but a field: one, going back to the physicist Ernst Mach, holds that this field originates in the nervous system; the other, going back to the psychologist James Gibson, holds that it originates in the environment and is revealed by movement through it. As we bring these scientific traditions to bear on what architects had been trying to articulate, new concepts emerge that could not have arisen within either discipline alone. Not science applied to architecture, and not architecture translated into science, but a new perspective that enriches both: an understanding of how the space around a moving person is structured for perception and for action. The work is unfolding. It can only continue as a collaboration of scientists and designers.