It is generally thought that science, by its very nature, must always progress. But this is not so. One day, fundamental science will come to an end. Not when we have discovered everything, but when we have discovered whatever is open to us to understand – which is not the same thing. Limitations as to what the human brain can comprehend, together with practical considerations to do with the need for ever more elaborate and expensive equipment, are likely to ensure that our knowledge will remain for ever incomplete. A further indication that the world will ultimately retain some of its mystery is suggested by evidence that in certain directions, scientific enquiry might already have come up against the boundaries of the knowable. Author and broadcaster Russell Stannard, himself a high-energy physicist and former Head of the Department of Physics and Astronomy at the Open University, introduces the general reader to the deepest questions facing us today – questions to do with consciousness, free will, the nature of space, time, and matter, the existence of extraterrestrial life, and why there should be a world at all. In doing so, he speculates as to whether some of these questions will never be answered.