The missile knows where it is at all times.
It knows this because it knows where it isn't
by subtracting where it is from where it isn't,
or where it isn't from where it is (whichever is greater),
it obtains a difference - or deviation.

the guidance subsystem uses deviations
to generate corrective commands to drive the missile
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 it was, is now the position that it isn't
....
...and by differentiating this from the algebraic sum of where it shouldn't be,
and where it was, it is able to obtain the deviation, and its variation, which is called error.

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