So what are systems, and what does it mean to think in them? The book defines systems as some set of things — cells, people, companies, economies, or whatever — “interconnected in such a way that they produce their own pattern of behavior over time.” The key insight in systems thinking is that those patterns of behavior are not driven or controlled by any of the individual actors within the system, but by the system’s own structure: the way that resources and information flows within the system.

Systems thinkers often model systems as structures of stocks and flows. Stocks are stores and quantities of some thing: either concrete like warehouse inventories, or abstract like knowledge on some particular topic. Flows are processes that change stocks over time.

To provide an example from the book, imagine a simple bathtub with a tap providing water, and a drain removing it:

We can abstract this into a diagram showing the stock (the amount of water in the bathtub) and the two flows which increase or decrease the amount of water: