Quite a good question and some very interesting responses. However, I take a more pragmatic view. For me, the steam engine was the most important invention in the past two thousand years. The steam engine freed man and beast from physical labor. No other invention had so many different and versatile uses. Man could cut down entire forests to feed sawmills to build cities, quarry stone, propel trains and ships to make the world a smaller place, power factories, and generate electricity. Agrarian society was over and industrialism reigned. Most importantly, the steam engine created more leisure time for mankind. No longer was leisure a pastime for the idle rich. The pursuit of leisure and the changes it created in society far outstripped the first 18 centuries. Without the steam engine, our society would be radically different from today.