is to ask what had to had to be known in order for a particular development to have occurred. It is doubtful, for example, if desktop computers of the power and flexibility we now have would have been possible without the invention of the silicon chip. This approach to emerging technologies produces a fan of necessary developments or, more aptly, a root system branching outwards as the historian moves backwards in time. Some of these roots are undoubtedly more important than others, some certainly more enabling. Consider the computer on my desk again. It is inconceivable that such a machine would have been possible without electricity. To be sure, Charles Babbage developed plans in the 1830s for what he called an analytical engine. His idea was that the machine would perform any arithmetical operation on the basis of instructions from punched cards, a memory unit in which to store numbers, sequential control, and most of the other basic elements of the present-day computer. The analytical engine was not built according to Babbage's specifications for another 150 years. Its mechanical components meant that it was bulky and the modern outgrowth of a Babbage machine would exclude both my desk and me from the room in which it sat - and it would do a fraction of what my liberating machine does. So, my candidate for the greatest invention of the last two thousand years is the harnessing of electricity. The first device that could store large amounts of electric charge was