Is the Motion From Left to Right Doing Any of the Work?

I'm not sure that he has avoided er giving a circular definition of time, presupposing the idea of time within the definition. But why do we call the left hand side the before? It's because it's what the motion reaches in time before it reaches the middle. So i'm afraid that he probably has got himself into a circle, even though he has tried to avoid it. Not a stupid circle, but i think he hasn't quite succeeded in avoiding it.