for the results of experiment. To abolish these principles, products of the scientific thought of several centuries, without finding a new explanation that encompasses them (in the same manner that the "Physics of principles" encompasses the "Physics of central forces"), is to claim that all of past physics has no intellectual value. Consequently, Poincaré had great confidence that the principles were salvageable. He said that it was the responsibility of mathematical physics to reconstitute those principles, or to find a replacement for them (the greater goal being to return the field to unity), given that it had played the main role in questioning them only after consolidating them to begin with. Moreover, it was the value of mathematical physics (in terms of the scientific method) which itself saw criticism, due to the implosion of certain theories. Two physics thus existed at the same time: the physics of Galileo and Newton, and the physics of Maxwell; but neither one was able to explain all the experimental observations that technical advances had produced. Electrodynamics of moving bodies The array of problems encountered concentrated on the electrodynamics of moving bodies. Poincaré swiftly proposed the idea that it is the ether modifying itself, and not the bodies acquiring mass, which came to contradict the older theories (based on a perfectly immovable ether). Overall, Poincaré shed light on the Zeeman effect, caused by discontinuous emissions of electrons. The problem of discontinuous matter forced the formulation of a minimally-destabilizing model of the atom. In 1913, Niels Bohr presented his atomic model which was based on the concept of electron orbits, and which explained spectroscopy as well as the stability of the atom. But, in 1905, the problem with all attempts to define the behavior of the microscopic world was that no one then knew if they needed to consider a similar model to the one known for the macroscopic objects (the model of classical mechanics), or if they should try to develop an entirely new model to give account of new facts. The latter idea, which was followed with the quantum theory, also implied definitively abandoning the unity already found in prior theories of mechanics. Future of mathematical physics Poincaré argued that the advancement of the physical sciences would have to consider a new kind of determinism, giving a new place to chance. And in effect, the history of twentieth century physics is marked by a