Relativity as a philosopher's idea: motion as pure kinematics

In the history of Western thought, the chapter on the relativity of motion is long, extraordinary and unfinished. A vivid thread of philosophical dogmatism runs right through it. One remarkable combination of facts is that philosophers from Descartes to the present day have seen it as obvious, even as trivial, that motion is somehow purely relative, whereas no theory of physics, with the doubtful exception of General Relativity, has been consistent with purely relative motion. It seems obvious that the concept of motion should be a very basic category of thought and, if not a primitive idea, then at least a very simple one. But the history of physics makes it clear that it is a rather intricate notion, inside physics at least. Does the open sentence ‘x moves’ express a cluster of concepts? This would be easier to accept were it not that the concepts of physics wind up inconsistent in a very strong sense with the ideas we might think of as the primitive ones. This is quite distinct from the better-known, but still dramatic way in which physics has overthrown our ideas of space and time. My aim in this chapter is to point out the more significant features of the philosophical idea of motion as it has changed from classical physics to Special and then to General Relativity.

But how does this interest tie in with the concerns of the book in general? No book about space can be complete without some discussion of the ancient question whether or not motion is relative. This is a huge topic.