Classical mechanics is a true theory of relativity. Motus inter corpora relativus tantum est; position and velocity are not properties of an object, only relative positions and velocities are observable. The equations of motion, called ‘second-order differential equations’, are the expression of this observation. It follows, too, that constant-velocity motion is the ‘ideal’, ‘natural’ or ‘inertial’ state of motion.

In Huygens's relativity, it makes no difference whether one is moving with constant velocity or standing still: according to the motus line, there is no way to decide between the two. The mere existence of an object is indistinguishable from its moving with constant velocity.

Unless, that is, our Universe has the property that some objects in it cannot stand still. In our everyday world, we are never aware of objects that cannot stand still with respect to us. It may take some effort, but it is perfectly possible to fly alongside a jet plane such that the velocity difference between you and the jet is zero.

But in a very surprising experiment in 1887, Michelson and Morley discovered that there is, in fact, something in our Universe that cannot stand still: light. Up to that moment, it was thought that light – of which Huygens had successfully argued that it is a shock-wave phenomenon – must move with respect to some carrier medium, like waves on the surface of water are moving with respect to the underlying water volume. To their immense surprise, Michelson and Morley found that light does not behave like that. The speed of light – traditionally called ‘c’ – is invariant. That is to say, light always moves with the same speed, no matter what the speed of the emitter or the receiver of the light is. Light rays cannot stand still with respect to anything. Huygens's assertion that the same object that some say to be at rest, may be said to move with respect to other objects turned out to be false after all, at least for light.

A little thought shows that, therefore, all light moves with the same speed. In summary: the speed of light is not relative, but absolute. Had he known this, Huygens would have written Motus inter corpora relativus tantum est, praeter lumen– movement between objects is relative in all aspects, with the exception of light.