ABSTRACT
The question ‘What is matter?’ is of the kind that is asked by metaphysicians, and answered in vast books of incredible obscurity. But I am not asking the question as a metaphysician: I am asking it as a person who wants to find out what is the moral of modern physics, and more especially of the theory of relativity. It is obvious from what we have learned of that theory that matter cannot be conceived quite as it used to be. I think we can now say more or less what the new conception must be.
