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Ought we to smile at the man and say that he errs in his

conclusion? I do not believe we ought to if we wish to remain

consistent; we must rather admit that his mode of grasping the

situation violates neither reason nor known mechanical laws. Even

though it is being accelerated with respect to the "Galileian

space" first considered, we can nevertheless regard the chest as

being at rest. We have thus good grounds for extending the

principle of relativity to include bodies of reference which are

accelerated with respect to each other, and as a result we have

gained a powerful argument for a generalised postulate of

relativity.