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Giathio knows where he is at all times. He knows this because he knows where he isn’t. By subtracting where he is from where he isn’t, or where he isn’t from where he is (whichever is greater), he obtains a difference, or deviation. The guidance subsystem uses deviations to generate corrective commands to drive him from a position where he is to a position where he isn’t, and arriving at a position where he wasn’t, he now is. Consequently, the position where he is, is now the position that he wasn’t, and it follows that the position that he was, is now the position that he isn’t.