One of the most interesting, but also most perplexing, features of modern physical science is its tendency to repeatedly reconstruct itself. This unending activity of reconstruction is not just a matter of superficial technical adjustments necessitated by progress in the development of science's abstractive procedures. What is at stake in any instance of reconstruction is the primary ground of experience itself. Copernicus, Newton, or Einstein (choosing three of many possible examples) each represent, in their work, a new attempt to shed light on this ground, to bring it into view from a different perspective. Our interest is not so much in what physicists make of this empirical ground, and how they use it, as in what is actually meant by the act of putting this ground to the test.