Here is the essence of both the Theory of Relativity and philosophy's development of it. Two men standing on two different planets moving at different speeds and at disproportionately different distances from the same object at the same instant of time, will differently perceive this object and differently estimate both its character and the measure of the forces working upon it. How can it be said that one of these results is wrong and the other right? Both are correct, for both must be what they are from their respective standpoints. But the same object and the same forces cannot at one and the same time possess contradictory measurements and properties. Therefore these men are not really dealing with it but with their own observations of it. On the other hand, two entirely different objects may produce two entirely similar sets of sense-impressions, as in the case of the meteor called shooting-star and a genuine star. Hence the things and forces in the world are not really the world-in-itself but what we individually see and experience as the world. All that we really know of them in the end is the picture which forms itself out of our sense-impressions, and this picture alone has genuine existence. Anything beyond it has only a supposed existence. But these impressions when thoroughly analysed are found to be only forms which the mind has unconsciously made for itself, just as a dreamer unconsciously makes his dream world for himself. The world of man's experience is always entirely relative to the individual man himself. All that he sees and smells lies wholly within his consciousness and not outside it.
-- Notebooks Category 19: The Reign of Relativity > Chapter 1: The Cosmos of Change > # 32
-- Perspectives > Chapter 19: The Reign of Relativity > # 1