equality or identity of two physical quantities is simpler than to ponder over the behaviour of the quantity which is their difference---distinguished though it may be by the important property of being incapable of existing! According to the views reached in this chapter the law of gravitation $G_{\mu\nu} = 0$ is not to be regarded as an expression for the natural texture of the continuum, which can only be forcibly broken at points where some extraneous agent (matter) is inserted. The differentiation of occupied and unoccupied space arises from our particular outlook on the continuum, which, as explained above, is such that the Hamiltonian derivatives of the principal invariant~$G$ stand out as active agents against the passive background. It is therefore the regions in which these derivatives vanish which are regarded by us as unoccupied; and the law $G_{\mu\nu} = 0$ merely expresses the discrimination made by this process. Among the minor points discussed, we have considered the speed of propagation of gravitational influence. It is presumed that the speed is that \PageSep{148} of light, but this does not appear to have been established rigorously. Any absolute influence must be measured by an invariant, particularly the invariant $B_{\mu\nu\sigma}^{\rho} B_{\rho}^{\mu\nu\sigma}$. The propagation of this invariant does not seem to have been investigated. The ordinary potential energy of a weight raised to a height is not counted \index{Potential energy}% as energy in our theory and does not appear in our energy-tensor. It is found superfluous because the property of our energy-tensor has been formulated as a general law which from the absolute point of view is simpler than the formal law of conservation. The potential energy and momentum~$\mf{t}_{\mu}^{\nu}$ needed if the formal law of conservation is preserved is not a tensor, and must be regarded as a mathematical fiction, not as representing any significant condition of the world. The pseudo-energy-tensor~$\mf{t}_{\mu}^{\nu}$ can be created and destroyed at will by changes of coordinates; and even in a world containing no attracting matter (flat space-time) it does not necessarily vanish. It is therefore impossible to regard it as of a nature homogeneous with the proper energy-tensor. \PageSep{149} \Chapter{V}{Curvature of Space and Time} \index{Curvature!of 4-dimensional manifold}% \Section{65.}{Curvature of a four-dimensional manifold} In the general Riemannian geometry admitted in our theory the~$g_{\mu\nu}$ may be any $10$~functions of the four coordinates~$x_{\mu}$.