Perspectival realists often appeal to the methodology of science to secure a realist account of the retention and continued success of scientific claims through the progress of science. However, in the context of modern physics, the retention and continued success of scientific claims are typically only definable within a mathematical framework. In this article, I argue that this concern leaves the perspectivist open to Cassirer’s neo-Kantian critique of the applicability of mathematics in the natural sciences. To support this criticism, I (...) present a case study on the conservation of energy in modern physics. (shrink)