“Recently I attended a workshop on the study of complexity at which two MIT computer scientists, Tom Toffoli and Norman Margolus, demonstrated the operation of an and gate on a computer monitor. Also watching the show was Charles Bennett of IBM, an expert on the mathematical foundations of computation and complexity. I remarked to Bennett that what we were watching was an electronic computer simulating a cellular automaton simulating a computer. Bennett replied that these successive embeddings of computational logic reminded him of Russian dolls.”