Why did the hunters in the Wealth of Nations exchange beavers for deer? - Joan Robinson

If knowledge develops as capital accumulates, there need be no tendency to diminishing returns, and with constant returns there can be no tendency for the rate of profit to fall (always assuming that the problem of effective demand as ruled out).

But, as soon as speculators become an important influence in the market, their business is to speculate on each others behaviour.

Until recently, Marx used to be treated in academic circles with contemptuous silence, broken only by an occasional mocking footnote. But modern developments in academic theory, forced by modern developments in economic life — the analysis of monopoly and the analysis of unemployment — have shattered the structure of orthodox doctrine and destroyed the complacency with which economists were wont to view the working of laissez-faire capitalism. Their attitude to Marx, as the leading critic of capitalism, is therefore much less cocksure than it used to be. In my belief, they have much to learn from him.