State of the System Diagram. png 
If you’re about to take a bath, you have a desired water level in mind (your goal). You plug the drain, turn on the faucet, and watch until the water rises to your chosen level (until the discrepancy between the goal and the perceived state of the system is zero). Then you turn the water off.
If you start to get into the bath and discover that you’ve underestimated your volume and are about to produce an overflow, you can open the drain for awhile, until the water goes down to your desired level.
Those are two **negative feedback loops**, or correcting loops, one controlling the inflow, one controlling the outflow, either or both of which you can use to bring the water level to your goal. Notice that the goal and the feedback connections are not visible in the system.
If you were an extraterrestrial trying to figure out why the tub fills and empties, it would take awhile to figure out that there’s an invisible goal and a discrepancy-measuring process going on in the head of the creature manipulating the faucets. But if you watched long enough, you could figure that out.
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