  Let \(C(t)\) be consumption at \(t\). It is a flow (units of consumption per moment). Similarly, we write \(K(t)\) for the stock of capital at \(t\). As \(dK(t)/dt\) is the rate of change in the capital stock at \(t\), it is “net investment at \(t\),” which too is a flow. And because the capital stock is assumed not to depreciate, gross investment equals net investment.