  Taking as a hypothesis a form of the labour theory of value, and $without$
$assuming$ $equilibrium$, we derive an equation that yields the profit-rate
$\pi$ as a function of time. For a mature economy, $\pi(t)$ reduces to the
product of two factors: ($i$) a certain $retarded$ $average$ of the sum of the
growth-rates of productivity and of the size of the labour-force measured by
hours worked, and ($ii$) the ratio of the current rate of surplus value to its
own retarded average. We also suggest an empirical test of the equation.
