In contrast to the discovery of new laws, processes, or properties, one of the hallmarks of the discovery of localizable natural objects such as we have been discussing in this volume is an almost irresistible temptation to classify them. A new object will barely have been discovered before the human mind tries to determine where it “fits” in the order of things already known. This is undoubtedly an ancient instinct, and one of the basic characteristics of a developing and even a mature science, as basic to astronomy as it is to biology, geology, chemistry, and physics. To put it another way, the recognition of new classes and their classification is an integral part of natural history, and a particularly challenging activity when the discovery involves a new class of objects. The origin of the categories in which we think is a question rarely raised except in the rarefied reaches of philosophy, but it goes to the core of discovery. In discovery it may flare briefly and sporadically, as in the case of Pluto, or at more sustained length as in the case of stellar luminosity types. But once routine, the origin of the categories is often forgotten, even as they determine the way we think of things.