The indeterminism associated with the quantum theory has been considered, as well by philosophers as by physicists, as the strongest deviation from classical physics that can be imagined. I do not believe that this judgment is defensible. It rather seems to me that this deviation is not as great as it seems at first sight, because there exist developments in classical physics which allow one to suppose that determinism is not the last word of the man of science regarding that which concerns natural phenomena. Thus, some other results of quantum physics contradict the classical conception of the physical world to a greater degree than does the abandonment of strict causality. I wish to speak of the change in the concept of matter, of the substance which makes up physical bodies, of a profound change which has resulted from the discovery of M. de Broglie regarding a certain equivalence of waves and particles and which has completely overturned philosophical theories (which had) come forth from classical science, as well as from our everyday experiences. Permit me, by way of introduction, to tell you about these developments from a comparative point of view, that is to say, by comparing them with classical ideas from which they have parted, and which they have transformed into a new picture of the physical world. It is only after this study of the philosophical background that we will be able to understand the significance of the discoveries of physicists during the thirty years elapsed since the first publications of M. de Broglie, and that we will be in shape to examine some other consequences of the new physics, consequences comparable if not superior to those which I have just cited.