When I received the invitation to give the Adamson Memorial Lecture I felt considerable hesitation about accepting it. I felt there was some incongruity in a lecture founded in memory of a great master of Metaphysics being given by one who had no qualifications to speak on that subject. I was reassured however when I remembered how wide were Professor Adamson's sympathies with all forms of intellectual activity and how far reaching is the subject of Metaphysics. There is indeed one part of Physical Science where the problems are very analogous to those dealt with by the metaphysician, for just as it is the object of the latter to find the fewest and simplest conceptions which will cover mental phenomena, so there is one branch of physics which is concerned not so much with the discovery of new phenomena or the commercial application of old ones, as with the discussion of conceptions able to link together phenomena apparently as diverse as those of light and electricity, sound, and mechanics, heat and chemical action. To some men this side of Physics is peculiarly attractive, they find in the physical universe with its myriad phenomena and apparent complexity a problem of inexhaustible and irresistible fascination.