The concepts which now prove to be fundamental to our understanding of nature-a space which is finite; a space which is empty, so that one point [of our 'material' world] differs from another solely in the properties of space itself; four-dimensional, seven- and more dimensional spaces; a space which for ever expands; a sequence of events which follows the laws of probability instead of the law of causation-or alternatively, a sequence of events which can only be fully and consistently described by going outside of space and time-all these concepts seem to my mind to be structures of pure thought, incapable of realisation in any sense which would properly be described as material.