The pigeon uses deviations to generate corrective commands to drive itself from a position where it is to a position where it isn't and, arriving at a position

The pigeon uses deviations to generate corrective commands to drive itself from a position where it is to a position where it isn't and, arriving at a position where it wasn't, it now is. Consequently, the position where it is, is now the position that it wasn't, and it follows that the position that it was is now the position that it isn't. In the event that the position that it is in is not the position that it wasn't, the system has acquired a variation; the variation being the difference between where the pigeon is and where it isn't.