cycle, the water cycle, and Earth's energy budget. Thus stocks and flows are the basic building blocks of system dynamics models. Jay Forrester originally referred to them as "levels" rather than stocks, together with "rates" or "rates of flow". A stock (or "level variable") in this broader sense is some entity that is accumulated over time by inflows and/or depleted by outflows. Stocks can only be changed via flows. Mathematically a stock can be seen as an accumulation or integration of flows over time – with outflows subtracting from the stock. Stocks typically have a certain value at each moment of time – e.g. the number of population at a certain moment, or the quantity of water in a reservoir. A flow (or "rate") changes a stock over time. Usually we can clearly distinguish inflows (adding to the stock) and outflows (subtracting from the stock). Flows typically are measured over a certain interval of time – e.g., the number of births over a day or month. Synonyms Examples Accounting, finance, etc. Other contexts Calculus interpretation If the quantity of some stock variable at time is , then the derivative is the flow of changes in the stock. Likewise, the stock at some time is the integral of the flow from some moment set as zero until time . For example, if the capital stock is increased gradually over time by a flow of gross investment and decreased gradually over time by a flow of depreciation , then the instantaneous rate of change in the capital stock is given by where the notation refers to the flow of net investment, which is the difference between gross investment and depreciation. Example of dynamic stock and flow diagram Equations that change the two stocks via the flow are: List of all the equations, in their order of execution in each time, from time = 1 to 36: History The distinction between a stock and a flow variable is elementary, and dates back centuries in accounting practice (distinction between an asset and income, for instance). In economics, the distinction was formalized and terms were set in , in which Irving Fisher formalized capital (as a stock). Polish economist Michał Kalecki emphasized the centrality of the distinction of stocks and flows, caustically calling economics "the science of confusing stocks with flows" in his critique of the quantity theory of money (circa 1936, frequently quoted by