in laboratories and elsewhere. Many of these discoveries gave substance to important theories. Other discoveries could not be explained satisfactorily - either they had only been occasionally observed, or they were inconsistent with the new and emerging theories. At the beginning of the 20th century, the unifying principles were thrown into question. Poincaré explains some of the most important principles and their difficulties: The principle of conservation of energy (which he called Mayer's principle) - the discovery of radium and radioactivity posed the problem of the continuous (and seemingly inexhaustible) energy emission of radioactive substances. The principle of entropy (which he called Carnot's principle) - Brownian motion seemed to be in opposition to the second law of thermodynamics. Newton's Third Law (which he called Newton's principle) — This law seemed to conflict with the laws of electrodynamics proposed by Maxwell, and with the ether theory he had proposed to explain them. The principle of conservation of mass (which he called Lavoisier's principle) — the consideration of movements at a speed close to that of light posed a problem for this principle; this is again an electrodynamic problem : the mass of a body in such a state of motion is not constant. the principle of relativity . Finally, he added the principle of least action. At the beginning of the twentieth century, the majority of scientists spoke of Poincaré's "diagnosis" concerning the crisis of the physical principles. In fact, it was difficult to do otherwise: they had discovered experimental facts which the principles could not account for, and which they evidently could not ignore. Poincaré himself remained relatively optimistic regarding the evolution of physics with respect to these severe experimental difficulties. He had little confidence in the nature of principles: they were constructed by physicists because they accommodate and take into account a large number of laws. Their objective value consists in forming a scientific convention, in other words in providing a firm foundation to the basis on which truth and falsehood (in the scientific meaning of the words) are separated. But if these principles are conventions, they are not therefore totally dissociated from experimental fact. On the contrary, if the principles can no longer sustain laws adequately, in accordance with experimental observation, they lose their utility and are rejected, without even having been contradicted. The failure of the laws entails the failure of the principles, because they must account