[Excerpt from the afterword of Engineering Control Systems, by Sven Shird]
There's a metaphor here, you know. Typically, the objective of control theory is to monitor the output of a system and compare it with the desired output (the reference signal). The difference between actual and the desired outputs (the error signal) is applied as feedback to the input of the system, to bring the actual output closer to the reference. Good control systems - and good engineers - learn from the past.