Our fourth condition, which is a condition not so much of stability as of alternate recession and recovery, is merely based on the presumption that capital assets are of various ages, wear out with time and are not all very long lived; so that if the rate of investment falls below a certain minimum level, it is merely a question of time (failing large fluctuations in other factors) before the marginal efficiency of capital rises sufficiently to bring about a recovery of investment above this minimum.