It is easy to refute this argument — correctly — with the benefit of experience, especially after the financial crisis which began in 2008. This crisis has shown very clearly that stock and futures prices by no means contain all relevant information and people don't always anticipate value correctly. And when people choose to believe government, such as when the chancellor says that all private bank accounts will be guaranteed, which is totally unfeasible, the rationality assumption also seems less convincing. Keynes knew why — from personal bad experience. He lost and earned back a lot of money, both on his own account and for King's College, whose investments he managed. He did, however, die a multimillionaire.

In his General Theory, Keynes describes the process that takes place in financial markets as a "beauty contest". Just as you don't win a beauty contest because you're the most beautiful person by any objective standard, you don't make money on the stock exchange because you identify the stocks of the most successful companies. It is not enough to scrutinise balance sheets and production technologies. If you manage to make a fortune, you do so because you have correctly anticipated the evaluation of stocks by others, and acted accordingly. This subjective valuation, Keynes thought, can be miles away from any objective standard, simply because people aren't rational and there is such a thing as a herd effect. We tend to do what others do because we hope to profit from the "wisdom of crowds" and also because that's what determines prices. But it can be misleading and destructive. 

Hayek didn't share Keynes's mixed experience with the financial markets, but he would have agreed that there is no such thing as an objective value of a stock. Hayek was a subjectivist. And he would not have understood why a rationally expected price should be one that already incorporates all relevant information. In his view, the economic problem consisted not just of an optimal allocation of scarce resources, but was rather a more generalised problem of social interaction. In that sense, it has to do with dealing with the "knowledge problem", our natural lack of all-encompassing knowledge, in such a way that a maximum of the bit of imperfect, decentralised, local knowledge that we individually possess could be gathered and communicated. In the marketplace, this happens as we react to prices. On the basis of our personal assessment of relative scarcity, we both take in and send out local information, and this action in itself becomes an element of newly-generated information for ourselves and for others. In short, Hayek's point is that this interaction has to take place effectively in order to determine a price, which is thus a subjective and interactive matter. It cannot be avoided, it cannot be circumvented, there is no shortcut. There is no other way of knowing what its level should be. 

Keynes did grasp this essential part of the Hayekian argument but couldn't be bothered with it. Nothing reflects this more clearly than his famous statement about the long term. In his Tract on Monetary Reform, which came out in 1923, he writes: "The long run is a misleading guide to current affairs. In the long run we are all dead. Economists set themselves too easy, too useless a task if in tempestuous seasons they can only tell us that when the storm is long past the ocean is flat again."