mbles he once admired. (Poincaré 1890, XVI) The epistemological value of the theorem is emphasized by Poincaré, who sees in it an explanation of the indecision between rival theories present in historical cases: The preceding is confirmed by the history of all the parts of physics; in optics, for example, Fresnel believes the vibration perpendicular to the plane of polarization; Neumann regards it as parallel to this plan. We have long sought an "experimentum crucis", which enabled us to decide between these two theories, and we could not find it. All these facts are easily explained by the properties of the Lagrange equations which I have just recalled. (Poincaré 1890, XIV)12 The explanations that postulate unobservable quantities are therefore a cause of underdetermination of theories. Also, the theorem highlights the abstract dynamics, favored by some of the British physicists. Among the old theoretical ensembles we find the molecular physics of Laplace. Poincaré establishes a close relationship between the tradition of French mathematical physics and the difficulties in understanding Maxwell's works: The first time that a French reader opens Maxwell's book, a feeling of uneasiness, and often even mistrust, mixes at first with his admiration. (...) Why do the ideas of the English scholar have so much difficulty in acclimatizing among us? It is no doubt that the education received by most enlightened Frenchmen disposes them to taste precision and logic before any other quality. The ancient theories of mathematical physics gave us a complete satisfaction in this respect. All our masters, from Laplace to Cauchy, proceeded in the same manner. Starting from clearly stated hypotheses, they deduced all the consequences with mathematical rigor, and then compared them with experience. (Poincaré 1890, V-VI) Poincaré emphasizes here the style of presentation, the logic and precision favored by the training of French physicists. He also recognizes the persistence of the Laplacian program, which contains an ontology 