Before solving the problem, however, Einstein had been led to recognise that space of itself was not fundamental. The fundamental continuum whose non-Euclideanism was to be investigated was therefore not one of space but one of Space-Time, a four-dimensional amalgamation of space and time possessing a four-dimensional metrical field governed by the matter distribution. Einstein accordingly applied Riemann’s ideas to space-time instead of to space, and attempted to explore the geometry of space-time by a purely rational co-ordination of known empirical facts. He discovered that the moment we substitute space-time for space (and not otherwise), and assume that free bodies and rays of light follow geodesics no longer in space but in space-time, the long-sought-for local variations in geometry become apparent. They are all around us, in our immediate vicinity; and yet we had never realised it. We had called their effects gravitational effects, ascribing them to forces foreign to the geometry of the extension, and never suspecting that they were the result of those very local variations in the geometry for which our search had ever been vain even though we had extended our observations to the depths of the universe. Indeed, it may be said that the theory of relativity is the theory of the space-time metrical field. While we are on the subject, we may mention that this mysterious metrical field which moulds both space and time appears to be conditioned entirely by the matter of the universe. Such at least are the conclusions which the existence of Einstein’s cylindrical universe would suggest. The problem is, however, still extremely obscure. It is still possible to believe with Eddington and de Sitter that the metrical field or space-time ether-structure might subsist in the absence of matter, contrary to the views of Riemann.