It's difficult for the average person to grasp just how much information can be packed into a city covering ten thousand square miles with a population density equal to that of Manhattan. How long would it take the hypothetical Man From Mars to investigate New York or London if he had only the City to work with, if he found them just as they stand except that the inhabitants had vanished? The technological level of the aliens could not be said to be either "above" or "below" that of Man: it could only be said to be "different." It was as if the two cultures complemented each other; the areas of knowledge which the aliens had explored seemed to be those which Mankind had not yet touched, while, at the same time, there appeared to be many levels of common human knowledge which the aliens had never approached. From the combination of the two, whole new fields of human thought and endeavor had been opened. No trace of the alien spaceships had been uncovered, but the anti-gravitational devices in their aircraft, plus the basic principles of Man's own near-light-velocity drive had given Man the ultralight drive. Their knowledge of social organization and function far exceeded that of Man, and the hints taken from the deciphered writings of the aliens had radically changed Man's notions of government. Now humanity could build a Galactic Civilization--a unity that was neither a pure democracy nor an absolute dictatorship, but resulted in optimum governmental control combined with optimum individual freedom. It was _e pluribus unum_ plus. Their technological writings were few, insofar as physics and chemistry were concerned. What there were turned out to be elementary texts rather than advanced studies--which was fortunate, because it had been through these that the cultural xenologists had been able to decipher the language of the aliens, a language that was no more alien to the modern mind than, say, ancient Egyptian or Cretan. But without any advanced texts, deciphering the workings of the thousands of devices that the aliens had left behind was a tedious job. The elementary textbooks seemed to deal with the same sort of science that human beings were used to, but, at some point beyond, the aliens had taken a slightly different course, and, at first, only the very simplest of their mechanisms could be analyzed. But the investigators learned from the simpler mechanisms, and found themselves able to take the next step forward to more complex ones. However, it still remained a fact that the majority of the devices were as incomprehensible to the investigators as would the function of a transistor have been to James Clerk Maxwell.