with lots of machine parts. Furthermore, imagine that this machine is a universal constructor: given a description of any other machine, it will paddle around the pond until it locates the proper parts, which it will then use to assemble that machine. In particular, given a description of itself, it will construct a copy of itself. Now, that is almost self-reproduction, said von Neumann—but not quite. The newly created copy of the first machine will have all the right parts, but it won't have a description of itself, which means that it won't be able to make any further copies of itself. So
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