The law only applies when you need to control the system exactly. No real-world controllers attempt that. Intuitively, the theorem says that to control a syste

The law only applies when you need to control the system exactly. No real-world controllers attempt that. Intuitively, the theorem says that to control a system exactly you have to see any external disturbances coming in advance, predict their effect (which requires knowing the entire state of the system) and generate control inputs to counteract them. But practical controllers usually don't bother with this, and just apply negative feedback based on the observed state. For instance, to steer a ship exactly you need to know all the waves and wind coming at it in the future. But to steer it adequately, you can just look at the heading and apply a low-pass filter to generate rudder commands. (The low-pass filter might require state consisting of 2-3 numbers). Or to control data transmission over the internet exactly you need to know the size and timing of everyone else's data packets, but you can do an adequate job with TCP just keeping track of ACKed packets.