By an “organism” I mean a living thing, in the most ordinary sense. There is an “organic theory of nature” in which the term “organism” is extended so as to include atoms, molecules, crystals, colloidal micelles, etc.; such constellations of parts have been called “inorganic organisms” by Driesch. I shall regard them as the “results of organization.” As we know them they are “models” (variant with the progress of physics), and they exist in the minds of physicists just as differential equations are in the minds of mathematicians. They express arrangements just as a trade union (an organization) is an arrangement, and just as artifacts, such as the nest of a chaffinch or the sleeping-bag of an Arctic explorer, are expressions of arrangements.