At first glance the argument looks convincing. It is, however, false. It rests upon a sequence of logical steps which you cannot take in the real world. Investment is a process of production that takes place at one point in time. The cheapening of further investment as a result of improved production techniques occurs at a later point in time. The two things are not simultaneous.44
There is an old saying, "You cannot build the house of today with the bricks of tomorrow." The fact that the increase in productivity will reduce the cost of getting a machine in a year's time does not reduce the amount a capitalist has to spend on getting it today. And if some other capitalist buys the cheaper machine, it immediately reduces the value of the machine owned by the first capitalist. While the new capitalist might be able to turn out goods in a more profitable way, the first capitalist has to deduct from his profits the loss in the value of his machine.45
When capitalists measure their rates of profit they are comparing the surplus value they get from running plant and machinery with what they spent on acquiring it at some point in the past-not what it would cost to replace it today. The point has added importance when it is remembered that the real process of capitalist investment takes place in such a way that the same fixed constant capital (machines and buildings) is used for several cycles of production. The fact that the cost of the investment would be less if it took place after the second, third or fourth round of production does not alter the cost before the first round of production.
The alleged disproof of Marx arises, as does the so-called transformation problem, from applying simultaneous equations to processes taking place through time. Simultaneous equations, by definition, assume simultaneity, with no passage of time.
The decline in the value of their invested capital certainly does not make life any easier for the capitalists. To survive in business they have to recoup, with a profit, the full cost of their past investments, and if technological advance means these investments are now worth, say, half what they were previously, they have to pay out of their gross profits for writing off that sum. What they have gained on the swings they have lost on the roundabouts, with "depreciation" of capital causing them as big a headache as a straightforward fall in the rate of profit.46
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Understanding the System: Marx and Beyond