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Disturbance

In PCT, anything that affects the state of a control system's controlled variable, other than the system's own actions, is called a  disturbance,  d.  A perceptual control system knows, and controls, the states of some of its own perceptions. It does not know the objective state of the environment. When its controlled perception is disturbed by something in the environment, the system acts to eliminate the effects of the disturbance, but the system does not know, nor does it need to know, the source, or sources, of the disturbing effects.

Sometimes, wind, and holes in the road, and tires that are out of balance, and a host of other influences, all add up to create a mismatch between a driver's perception of "where the car is on the road right now," and "where the driver intends the car to be on the road."  When there is a mismatch, the driver turns the steering wheel, initiating a series of events that result in her perceiving the car where she intends to perceive it on the road. She does not know, and does not need to know, the planetary and atmospheric events that produced the wind that deflected the car from her intended path. She does not know, and does not need to know, all of the events that led up to the tires on her car being out of balance, so that the imbalance added to the deflection of the car from the position she intended to perceive. All the driver knows is where she intends the car to be, and where it is right now. She acts to  prevent a mismatch between where it is, and where it ought to be, and when a mismatch does occur, she acts to eliminate it.

Like any other perceptual control system, the driver acts to eliminate, or balance, the net effect of all environmental factors that would otherwise change the state of a controlled perception. It is not necessary for a control system to perceive the  sources of disturbances to its controlled perceptions. All that is necessary is that the system be able to act to cancel the net effect of all of the disturbances that affect the environmental variables that affect its own controlled perceptions.

Some disturbances originate from inanimate sources, like weather changes, holes in the road, soil erosion, wells that run dry, failure of components in mechanical or electrical systems, and so on.  Other disturbances originate from the actions of other people -- other perceptual control systems. Sometimes one person deliberately acts to disturb another person's controlled perceptions,  but many times one person unknowingly disturbs the other person, by way of incidentally affected variables. The chapters reproduced on Ed Ford's site, at the   Section on PCT, contain several examples of how one person, acting to control his or her own perceptions, can incidentally disturb another person's controlled perceptions.