Since the turn of the century, however, a double upheaval has occurred, the formulation of the quantum theory and the theory of relativity, providing the ground for the development of modern physics. These theories issued from the problems of light that, in their strict forms, could not be assimilated by Newtonian physics. Before the turn of the century the wave theory had been victorious over the emission theory, and an hypothetical ether was assumed which was intended ultimately to represent absolute (...) space and to play a role similar to that of Anaximander's apeiron or Aristotle's prima materia. This last role seemed required since many philosophers of nature believed that they should see in matter and its elementary components the singularities, the exceptional peculiarities of the ether. However, since the ether held itself strictly incognito, Albert Einstein renounced it and declared that all systems in uniform, rectilinear motion are equivalent. The first portentous ontological consequence of this renunciation was that statements concerning spatial and temporal separation become relative. True relativity means, however, that objectivity and subjectivity enter equally into all statements, for to everything relative, as Max Planck once said, an absolute belongs. In this case the absolute was the combination of space and time into a four-dimensional spatio-temporal unity, which observers in different systems split into different space and time determinations. Our statements concerning space and time are thus neither merely subjective nor merely objective; rather a subject-object relation enters into them and makes them all relative. There is, to be sure, within the four-dimensional spatio-temporal unity a region of a spatial type within which the space-axes of the different observers lie, and the same is true for the temporal region. But even if there is no one universal time or simultaneity, temporality nevertheless plays its own role in contradistinction to spatiality, though both are expressible mathematically. We can say in the sense of critical realism that our spatial and temporal measurements are commensurate with the ordering relations of things and events. (shrink)