Now, a hundred years later, this guy comes along, Alan Turing, and in 1936, and invents the computer all over again. Now, of course, Babbage's machine was entirely mechanical. Turing's machine was entirely theoretical. Both of these guys were coming from a mathematical perspective, but Turing told us something very important. He laid down the mathematical foundations for computer science, and said, "It doesn't matter how you make a computer." It doesn't matter if your computer's mechanical, like Babbage's was, or electronic, like computers are today, or perhaps in the future, cells, or, again, mechanical again, once we get into nanotechnology. We could go back to Babbage's machine and just make it tiny. All those things are computers. There is in a sense a computing essence. This is called the Church–Turing thesis.