The philosophical implications of relativity theory will doubtless take a long time to work out. The four-dimensional universe of point-events is something that can be argued about but it is, to use an old-fashioned phrase, “inconceivable”. Mankind, excepting professional logicians, never remains content with the inconceivable. A purely logical conclusion is not enough; it has to be grasped imaginatively, by which I do not necessarily mean that it has to be pictured. To become familiar with a theory does not merely mean that one is able, as a form of mental wire-walking, to slip nimbly back and forth over the logical connections of the structure. It means taking it into oneself in some indefinable manner――becoming “intimate” with it. Only when a theory is “realized”, as we say, do we feel that we truly understand it. Ideas, points of view, that we were able to see only in flashes, become part of our normal intellectual equipment. The process may well be called a growth of consciousness. There are ideas which our consciousness, when it first approaches them is, as it were, too flabby to grasp. We first have to exercise our mental muscles. Every student of a line of thought such as mathematics, which is rather outside our normal preoccupations, becomes aware of an actual change in his mental powers. Notions so abstract that at first they seemed almost meaningless gradually become perfectly clear and permanent additions to one’s mental resources. Students of musical composition find that their capacity for mentally hearing a number of parts rapidly increases. In some cases it is almost as if a new faculty of the mind were born and developed.