In every age, the external scenery in which the human drama has been framed has remained pretty much the same. There have been fluctuations in climate and changes in terrain; and at times a great civilization, like that of the Mayas in Central America, has arisen where now only a thick net of jungle remains; but the hills around Jerusalem are the hills that David saw; and during the historic period the drowning of a city in the Netherlands or the rise of a shifting bank of real estate along the coast of New Jersey is little more than the wearing off of the paint or a crack in the plaster. What we call the material world constantly changes, it goes without saying: mountains are stript of trees and become wastes, deserts are plowed with water and become gardens. The main outlines, however, hold their own remarkably well; and we could have travelled better in Roman days with a modern map than with the best chart Ptolemy could have offered us. If the world in which men live were the world as it is known to the physical geographer, we should have a pretty simple time of it. We might follow Whitman’s advice, and live as the animals, and stop whining for all time about our sins and imperfections. What makes human history such an uncertain and fascinating story is that man lives in two worlds--the world within and the world without--and the world within men’s heads has undergone transformations which have disintegrated material things with the power and rapidity of radium. I shall take the liberty of calling this inner world our idolum (ido′lum) or world of ideas. The word “ideas” is not used here precisely in the ordinary sense. I use it rather to stand for what the philosophers would call the subjective world, what the theologians would perhaps call the spiritual world; and I mean to include in it all the philosophies, fantasies, rationalizations, projections, images, and opinions in terms of which people pattern their behavior. This world of ideas, in the case of scientific truths, for example, sometimes has a rough correspondence with what people call the world; but it is important to note that it has contours of its own which are quite independent of the material environment. Now the physical world is a definite, inescapable thing. Its limits are narrow and obvious. On occasion, if your impulse is sufficiently strong, you can leave the land for the sea, or go from a warm climate into a cool one; but you cannot cut yourself off from the physical environment without terminating your life. For good or ill, you must breathe air, eat food, drink water; and the penalties for refusing to meet these conditions are inexorable. Only a lunatic would refuse to recognize this physical environment; it is the substratum of our daily lives.