The greater the rate at which productivity (and hence future output) grows over time, the smaller (ceteris paribus) should be the amount of present labour invested as stored-up labour, and the less expensive should be the investment-projects that are chosen. It might be held to afford a principle of choice between investment-projects in the limiting case where no growth of productivity over time was to be expected and future income was likely to be no greater than present income. For example, decisions about the capital-intensity of various projects (according to the criterion we are considering) might result in a total investment (as the aggregate of all the separate decisions of each industry) larger than had been originally decided upon. To be worth while, a given investment-project must suffice to yield a net addition to annual output of the relative amount (as a ratio to its original cost), as well as enough to allow for its eventual replacement (or perpetual maintenance).