Stocks and Flows
Every system runs on stocks — the things that accumulate — and flows, the rates that fill or drain them. Picture a bathtub: the water is the stock, the tap and the drain are the flows.
A stock is anything that piles up over time: water in a tub, money in an account, trust in a relationship, carbon in the air, people in a city. A flow is a rate that changes it — water in, water out; deposits and withdrawals; births and deaths.
The crucial rule is that a stock changes only through its flows, and only as fast as they allow. You cannot teleport the water level; you can only open the tap wider or the drain smaller. Stocks integrate their flows over time.
Seeing this transforms how you read a system. Most arguments confuse the level with the rate — the debt with the deficit, the weight with the diet. Separate the two and a confusing system suddenly has a plumbing diagram you can actually work with.
Find the stock, then the taps
Separate levels from rates
For any system, list what accumulates (the stocks) and what changes them (the flows). Ask of every number: is this a level or a rate? The two behave completely differently.
- Stock: what's piled up right now
- Flow: how fast it's rising or falling
Draw the bathtub
Sketch the stock as a container with an inflow pipe and an outflow drain. This one diagram is enough to reason about savings, inventory, reputation, or a reservoir.
Remember stocks move only through flows
You can't change a stock directly — only its inflows and outflows. Any plan to raise or lower a level must name which tap it turns.
Master the bathtub and you have the grammar every dynamic system is written in.