I n the First Part we have been dealing with the gradual development and modification of the traditional scientific concepts of Space, Time, and Motion, within the region of Physics. These concepts were taken over by science from educated common-sense, and we have been tracing the process of clarification and definition which they have undergone at the hands of scientists in pursuit of their own business. At two places only have we deliberately gone outside the range of ordinary scientific reflection. The first was where we explained the Principle of Extensive Abstraction, and tried to justify by its means what mathematical physicists take for granted, viz., the application of geometry and mechanics, stated in terms of points, instants, and particles, to a world of extended objects and non-instantaneous events. The second was where we dealt with the general problem of Time and Change, and tried to defend their reality against the very plausible ob­ jections which have been made to them by certain philosophers.