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INTRODUCTION
separate notions of space and time by a unified notion of space-time. In particular, Minkowski showed that the Lorentz transformations of special relativity correspond to a rotation of axes in space-time. He showed how natural the kinematics of special relativity can seem, as opposed to Newtonian kinematics, in which, in effect, we should rotate the time axis without correspondingly rotating the space axes. Since the theory of relativity it has become a commonplace to regard the world as a four dimensional space-time manifold. Nevertheless, even in the days of Newtonian dynamics, there was nothing to prevent taking this view of the world, even though it would not have been as neat as it is in relativity theory. If we pass to the four-dimensional way of looking at things, it is important not to be confused about certain conceptual matters. Confusion will arise if we mix the tenseless way of talking appropriate to the four-dimensional picture with our ordinary way of talking of things as enduring substances, “the permanent in change."
Isaac Newton held to an absolute theory of space and time, whereas his comtemporary Leibniz argued that space and time are merely sets of relations between things which are in space and time. Newton misleadingly and unnecessarily expressed his absolute theory of time in terms of the myth of passage, as when he confusingly said, "Absolute, true and mathematical time, of itself and from its own nature, flows equably without relation to anything external.” The special theory of relativity has made it impossible to consider time as something absolute; rather, it stands neutrally between absolute and relational theories of space time. The question as between absolute and relational theories of space-time becomes especially interesting when we pass to the general theory of relativity: According to this theory, the structure of spacetime is dependent on the distribution of the matter in the universe. In most forms of the theory there is nevertheless a residual space-time structure which cannot be thus accounted for. A curvature is usually attributed to space-time even in the complete absence of matter, and the inertia of a body, according to this theory, depends in part on this cosmological contribution to the local metrical field and hence not solely on the total mass of the universe, as a purely relational theory would require. Research on this question is still going on, and until it has been decided, Mach's principle (as Einstein called it), according to which the spatio-temporal structure of the universe depends entirely on the distribution of its matter, will remain controversial. But even if Mach's principle were upheld, it might still be possible to interpret matter, in a
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