Summary of Systems Principles

Reproduced from pp.188-191 of Thinking in Systems

A system is more than the sum of its parts.
Many of the interconnections in systems operate through the flow of information.
The least obvious part of the system, its function or purpose, is often the most crucial determinant of the system’s behavior.
System structure is the source of system behavior. System behavior reveals itself as a series of events over time.

A stock is the memory of the history of changing flows within the system.
If the sum of inflows exceeds the sum of outflows, the stock level will rise.
If the sum of outflows exceeds the sum of inflows, the stock level will fall.
If the sum of outflows equals the sum of inflows, the stock level will not change — it will be held in dynamic equilibrium.
A stock can be increased by decreasing its outflow rate as well as by increasing its inflow rate.
Stocks act as delays or buffers or shock absorbers in systems.
Stocks allow inflows and outflows to be de-coupled and independent.

A feedback loop is a closed chain of causal connections from a stock, through a set of decisions or rules or physical laws or actions that are dependent on the level of the stock, and back again through a flow to change the stock.
Balancing feedback loops are equilibrating or goal-seeking structures in systems and are both sources of stability and sources of resistance to change.
Reinforcing feedback loops are self-enhancing, leading to exponential growth or to runaway collapses over time.
The information delivered by a feedback loop — even nonphysical feedback — can affect only future behavior; it can’t deliver a signal fast enough to correct behavior that drove the current feedback.
A stock-maintaining balancing feedback loop must have its goal set appropriately to compensate for draining or inflowing processes that affect that stock. Otherwise, the feedback process will fall short of or exceed the target for the stock.
Systems with similar feedback structures produce similar dynamic behaviors.

Complex behaviors of systems often arise as the relative strengths of feedback loops shift, causing first one loop and then another to dominate behavior.
A delay in a balancing feedback loop makes a system likely to oscillate.
Changing the length of a delay may make a large change in the behavior of a system.

System dynamics models explore possible futures and ask "what if" questions.
Model utility depends not on whether its driving scenarios are realistic (since no one can know that for sure), but on whether it responds with a realistic pattern of behavior.

=== Constraints on Systems

In physical, exponentially growing systems, there must be at least one reinforcing loop driving the growth and at least one balancing loop constraining the growth, because no system can grow forever in a finite environment.
Nonrenewable resources are stock-limited.
Renewable resources are flow-limited.

There are always limits to resilience.
Systems need to be managed not only for productivity or stability, they also need to be managed for resilience.
Systems often have the property of self-organization — the ability to structure themselves, to create new structure, to learn, diversify, and complexify.
Hierarchical systems evolve from the bottom up. The purpose of the upper layers of the hierarchy is to serve the purposes of the lower layers.

Many relationships in systems are nonlinear.
There are no separate systems. The world is a continuum. Where to draw a boundary around a system depends on the

purpose of the discussion.

At any given time, the input that is most important to a system is the one that is most limiting.
Any physical entity with multiple inputs and outputs is surrounded by layers of limits.
There always will be limits to growth.
A quantity growing exponentially toward a limit reaches that limit in a surprisingly short time.
When there are long delays in feedback loops, some sort of foresight is essential.
The bounded rationality of each actor in a system may not lead to decisions that further the welfare of the system as a whole.

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