This symposium will explore physicists at work in the nineteenth and twentieth centuries doing what we recognize now as philosophy-metaphysical and conceptual work. Often, but not always, this was recognized as philosophical or “foundational” work by physicists themselves. This work blurs traditional disciplinary boundaries between physics and philosophy, and challenges the picture of modern physical science-particularly American science-as being characterized by an anti-philosophical pragmatism. The symposium will contribute detailed studies to a growing literature of technically- and philosophically-sophisticated histories of science, particularly physical science. Participants will ask questions such as: How did Helmholtz’s empiricism play out in his laboratory? How was “classical physics” created alongside “modern physics” in optics? How did debates about the photon continue through post-war physics? How were concepts of time reconfigured in atomic clock making? What was “foundations of physics” in the twentieth century? And, how did logical concepts become physical concepts in the thermodynamics of computing? This symposium will contribute to creating a fruitful environment to discuss those questions and others through different historical and philosophical approaches.