A constructor is actually a limiting case, an unphysical limiting case of real physical objects. You can perform that with 99% accuracy or 99.9% accuracy, but not with 100% accuracy and 100% reliability. For any given machine that can convert hydrogen and oxygen into water imperfectly, there's another machine that one could build that could build it less imperfectly. But there is no machine that could built it with absolute perfection. Now, you can regard a human being as a highly imperfect universal constructor in that, given enough knowledge, a person could build a universal constructor that's better than itself. A universal constructor has to be perfectly obedient. And people and collections of people are disob