Fizeau’s experiment permits this verification and proves that the composition of velocities is Lobatchewskian, not Euclidean, and corresponds to the existence of a critical invariant velocity of 186,000 miles per second. It is true that in Fizeau’s experiment we are again dealing with the propagation of light, though no longer in vacuo, where its speed is invariant, but through water. It is precisely because its speed through water is reduced that the velocity of the water will affect the velocity of the light, thereby permitting us to test out the relativistic law of the composition of velocities. But once again the reason why light propagations enter into the majority of these tests is due solely to the fact that optical experiments permit of greater accuracy. Apart from this, there is no need to limit ourselves to experiments dealing with the propagation of light, whether in vacuo or through material bodies such as water. Any speed, be it the speed of sound or of a bullet shot from a passing train, should enable us to detect the same law of Lobatchewskian composition and thus allow us to demonstrate the existence of space-time. At all events, one point should be clear by now. Whatever method we follow, we can never get away from performing physical measurements of one kind or another. It is therefore meaningless to speak of defining the invariant critical velocity without reference to a measurement of the speed of light in vacuo, or more generally, without reference to physical measurements bearing on the phenomena which the existence of a finite invariant speed in the universe would necessarily entail. From this it follows that without physical measurements of one kind or another we have no means of establishing whether the world is one of separate space and time or one of space-time.