How many times does not positive science find itself obliged to modify and to readjust a system of long acquired notions, in order to make a place for new elements? This is a work often very laborious, but from which science never dreams of shrinking, knowing that it is made liable to it, so to speak, by definition, that is to say, that it is relative. Examples of this abound, not only in the history of physical and natural science, but even in that of so-called exact sciences. Do we not hear M. Poincaré declaring in accordance with Hertz, that given the system of Galileo and of Newton in mechanics it is impossible to give a satisfactory idea of mass and of force?[43] Thus the definitions, and even the laws, established by the positive sciences, are at every period approximations corresponding to the knowledge we have of facts. And as this knowledge can always be enriched the approximation may also become stricter, without ever reaching its confines. Leibnitz already said that the analysis of anything real reaches to infinity. This thought is with him, closely allied to the whole of his metaphysics. We find in Comte an expression in some way equivalent, although positive. He says, although the progress of the science of nature consists in substituting as much as possible the rational method to the experimental method, the limit can never be attained, we can never affirm that experience will not bring new elements which will oblige us to modify the edifice of science. The relativity of science thus serves to maintain an equal balance between the need of unity which comes from the understanding, and the inexhaustible diversity of the world of reality which this understanding studies. As a fact, then, positive science is always relative. Rightly, it cannot be otherwise, and this for two essential reasons. It depends necessarily upon “our organisation” and “our situation”[44] or, in other words, it is relative “both to the individual and to the species in its advance.”