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Module 1 of 16
Orientation · the grammar of systems
Every course on this platform reads something — a market, an adversary, a supply chain, an organization — as a system. This course is the grammar that reading depends on. It is built the way the rest of the academy is built: a small periodic table of elements that is provably complete, a hierarchy in which each rung is composed from the one below, a fixed template for every atom so that nothing is learned vaguely, and a thesis that the whole course is an argument for. The thesis is four words long — behaviour follows from structure — and by the end you will be able to take a behaviour you can see and draw the structure that must be producing it, then say where and when to push it.
The event-reactor
Notices the outcome — the stock-out, the price spike, the burnout, the collapse — and treats it as an event with a cause. Looks for the actor or the decision nearest to it. Pushes the lever closest to hand, usually a parameter: a budget, a target, a price, a headcount. Is surprised when the system pushes back, when the effect arrives late, when the fix produces a second problem somewhere else, and when the same event recurs a year later with different people in the chairs. Explains all of this as bad luck, bad people, or bad execution.
The structure-reader
Refuses to discuss the event until it is a line on a graph over time — because a single event is one point, and structure only shows in the pattern. Asks which stock is accumulating, which flows fill and drain it, and which feedback loop is running the show. Expects the delay, and therefore does not over-correct. Expects resistance, and therefore looks past the parameters to the rules, the information flows and the goal. Chooses the leverage point and the moment, acts once, and watches the graph rather than the headlines to learn whether it worked.
The difference between the two is not intelligence or experience; it is vocabulary. The event-reactor has words for actors and events. The structure-reader has words for stocks, flows, loops, delays, modes, archetypes and leverage — and, having the words, sees the things. This course is that vocabulary, taught to the letter.
Any dynamic system — a bathtub, a bank account, a population, a company, a reputation, a war — can be written down using exactly six kinds of thing. This is not a teaching simplification; it is the foundational claim of system dynamics, and it holds because the six are defined by role rather than by subject: anything that accumulates is a stock, anything that changes an accumulation is a flow, and so on. A diagram of a system contains these six and nothing else.
An accumulation measured at an instant: water in a tub, cash in an account, people in a company, trust in a relationship, carbon in the air. The system's memory; the only thing that can be in a state.
A rate measured over an interval: litres per minute, hires per month, revenue per quarter. The only way a stock changes. Where control lives.
A quantity computed from stocks, parameters and other auxiliaries — a gap, a ratio, a perceived value, a pressure. The system's arithmetic between sensing and acting.
A number the model holds constant — a goal, a capacity, a rate coefficient, a delay time. Where most people push first, and the weakest place to push.
A dependency: this quantity is used to set that rate. It carries no material. Chains of information links closing back on a stock are what make a feedback loop.
The source and sink clouds: what the model chooses not to explain. Every system has one; every boundary is a decision; most analytical errors are boundary errors.
The completeness argument, stated once. A system is a set of quantities that change over time. Any quantity either accumulates (a stock) or is a rate of change (a flow) or is computed from other quantities without accumulating (an auxiliary) or is held fixed (a parameter). The relationships between them are either material (a flow into or out of a stock) or informational (a quantity used in a computation — a link). What is left outside is the boundary. There is no seventh kind: a "delay" is a stock with a flow through it and a time constant; a "loop" is a closed chain of links; an "archetype" is a named arrangement of loops; a "behaviour mode" is what such an arrangement does over time. The rest of this course is those compositions, in order.
The ladder the sidebar follows. Elements compose into loops; loops produce a small number of behaviour modes; recurring arrangements of loops are archetypes; whole systems have properties no loop has alone; leverage is where in this structure to push; the analyst is the set of models, instruments and procedures that does the reading. Nothing on a higher rung is explained by anything but the rungs below it.
The persistent visual of this course is a pair of panels. On the left, a structure drawn in the six elements. On the right, the behaviour over time that this structure — and only structures like it — can generate. Reading a system is moving from right to left: you can see the behaviour, and you must infer the structure. Changing a system is moving from left to right: you alter the structure, and the behaviour follows, after a delay you should have predicted.
The twin panel in its simplest form. Left: a stock whose own level sets its inflow — a reinforcing loop. Right: the one behaviour that structure produces. Every module adds a structure on the left and its curve on the right until the full catalogue of shapes and their generators is in hand.
The load-bearing idea
Behaviour follows from structure. Not from people, not from luck, not from the last decision. The same structure produces the same behaviour whoever is inside it; change the people and keep the structure, and the behaviour returns. This is why the structure-reader never starts with "who" — and why, once the structure is drawn, the behaviour stops being a surprise.
Each module from the second onward dissects one rung into its atoms and treats every atom with the same six fields:
| Field | What it must contain |
|---|---|
| What | The definition, stated once and exactly, with what it is not |
| Structure | The mechanism — always with a diagram in the six-element notation, and the behaviour graph beside it where one exists |
| Technique | A numbered procedure you can run on a real case: how to find it, draw it, measure it, or use it |
| Properties | Its characteristic behaviour, strengths, failure modes and trade-offs |
| When / When not | The situations that call for it and the situations where reaching for it is a mistake |
| Where it fits | Its place in the hierarchy — what it is composed from, and what composes it |
Modules end with a side-by-side comparison of their atoms, the key insights, two exercises with answer keys, and a worksheet you copy into your own case. Two capstones run the whole ladder on complete cases; the second closes with the master equation and a self-assessment.
A hospital ward: patients are admitted at a rate the emergency department sets; patients are discharged when they recover, which takes on average four days; the ward has 30 beds; when occupancy passes 90 % the department slows admissions. Identify every stock, flow, auxiliary, parameter and information link, and say where the boundary has been drawn.
Hint: ask of each quantity — does it accumulate, does it change an accumulation, is it computed, or is it held fixed?
Module 2 takes the word system apart before any diagram is drawn: the six things that make a set of parts a system at all — element, interconnection, purpose, boundary, environment and hierarchy — and the technique for deciding, on a real case, where the system ends.
The six elements in their standard notation, arranged as the simplest complete system: a stock filled and drained by two flows, an auxiliary that compares the stock with a goal, and an information link that lets the comparison set the inflow. Every diagram in this course is made of these six shapes and no others.