In philosophy classes, we hear of the fabled struggles between theory T and T  ′, armored in their respective vestments of laws, observational consequences, and (if one follows David Lewis) “natural properties.” Eventually our curiosity (or conscience) becomes piqued, and we wonder how these literals can get cashed out in concrete doctrine. We anticipate that the “classical physics” of freshman experience will supply easy exemplars, but these expectations are soon confounded. At first sight, our subject's contours appear evident enough—worlds of point masses held together through action-at-a-distance forces monitored by Newton's three laws of motion—yet these simple outlines grow misty as we approach the shoreline more closely. We had presumed that a bevy of “force laws” capable of binding matter into stable configurations would emerge, but they never do, and unexpected interlopers appear in their stead (appeals to rigid bodies or malleable continua). And...

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