[From Bruce Abbott (2016.11.21.1245 EST)]
Here is a short passage from Chapter 1 of Jay W. Forrester’s book, Principles of Systems, 2nd preliminary edition (1973). One caution is in order: Forrester is using the word “control” in the sense of “having a dominating effect on,” not as we do, as the process whereby a variable is brought near a reference variable and kept there against disturbances. Forrester’s “control” is consistent with engineering usage: A “motor controller” is said to “control” the speed of a motor even when the “controller” operates without feedback. One operating as a control system would be termed a “negative feedback controller.” Now on to the quoted material from Forrester’s book:
1.3 Systems—Open and Feedback
Systems can be classified as “open” systems or “feedback” systems.
An open system is one characterized by outputs that respond to inputs but where the outputs are isolated from and have no influence on the inputs. An open system is not aware of its own performance. In an open system, past action does not control future action. An open system does not observe and react to its own performance. An automobile is an open system which by itself is not governed by where it has gone in the past nor does it have a goal of where to go in the future. A watch, taken by itself, does not observe its own accuracy and adjust itself—it is an open system.
A feedback system, which is sometimes called a “closed” system, is influenced by its own past behavior. A feedback system has a closed loop structure that brings results from past action of the system back to control future action. One class of feedback system—negative feedback—seeks a goal and responds as a consequence of failing to achieve the goal. A second class of feedback—positive feedback—generates growth processes wherein action builds a result that generates still greater action.
A feedback system controls action based on the results from previous action. The heating system of a house is controlled by a thermostat which responds to the heat previously produced by the furnace. Because the heat already produced by the system controls the forthcoming generation of heat, the heating system represents a negative feedback system that seeks the goal of proper temperature. A watch and its owner form a negative feedback system when the watch is compared with the correct time as a goal and is adjusted to eliminate errors. An engine with a governor senses its own speed and adjusts the throttle to achieve the preset speed goal—it is a negative feedback system. Bacteria multiply to produce more bacteria which increase the rate at which new bacteria are generated. In this positive feedback system the generation rate of new bacteria depends on the bacteria accumulated from past growth of bacteria.
Source: Forrester, J. W. (1968, 1973). Principles of systems, 2nd preliminary ed. Cambridge, MA: Wright-Allen Press. P. 1-7.