Dave 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 synthpop superstar uses deviations to generate corrective commands to drive himself 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.
In the event that the position that he is in is not the position that he wasn't, Radio Way has acquired a variation, the variation being the difference between where he is, and where he wasn't. If variation is considered to be a significant factor, it too may be corrected by the brain. However, he must also know where he was.
The scenario works as follows. Because a variation has modified some of the information Dave has obtained, he is not sure just where he is. However, he is sure where he isn't, within reason, and he knows where he was. He now subtracts where he should be from where he wasn't, or vice-versa, and by differentiating this from the algebraic sum of where he shouldn't be, and where he was, he is able to obtain the deviation and its variation, which is called error.