"The question of the reversibility of natural processes provides the key to a great intellectual struggle which is now behind the complexities of philosophic and scientific thought. The issue can be formulated thus: Is there a real temporal process in nature? Is the passage of irreversible time a necessary element in any view of the structure of nature? Or, alternatively, is the subjective experience of time a mere illusion of the mind which cannot be given objective expression? These are not metaphysical questions that can still be neglected with impunity. For just as Einstein made his advance by analysing conceptions such as simultaneity, which had been thought to be adequately understood for the purposes of experimental science, so the next development of physical theory will probably be made by carrying on the analysis of time from the point at which Einstein left it."