Since the 1970s economists have been engaged in a grand project. The project's objective is that macroeconomics should have microeconomic foundations. In everyday language, that means that what we say about big policy issues – growth and inflation, boom and bust – should be grounded in the study of individual behaviour. Put like that, the project sounds obviously desirable, even essential. I confess I was long seduced by it.
Most economists would claim that the project has been a success. But the criteria are the self-referential criteria of modern academic life. The greatest compliment you can now pay an economic argument is to say it is rigorous. Today's macroeconomic models are certainly that.
But policymakers and the public at large are, rightly, not interested in whether models are rigorous. They are interested in whether the models are useful and illuminating – and these rigorous models do not score well here.
Indeed, at an early stage of the project Robert Lucas, one of its principal architects, who received the Nobel prize for his contributions, developed what is known as the Lucas critique. He argued that ordinary standards of statistical validity should not be applied to the project's predictions. According to his colleague Thomas Sargent, Lucas was concerned that such tests rejected "too many really good models".
Economists, like physicists, have been searching for a theory of everything. If there were to be such an economic theory, there is really only one candidate, based on extreme rationality and market efficiency. Any other theory would have to account for the evolution of individual beliefs and the advance of human knowledge, and no one imagines that there could be a single theory of all human behaviour. Not quite no one: a few deranged practitioners of the project believe that their theory really does account for all human behaviour, and that concepts such as goodness, beauty and truth are sloppy sociological constructs.
But these people discredit themselves by opening their mouths. That people respond rationally to incentives, and that market prices incorporate information about the world, are not terrible assumptions. But they are not universal truths either. Much of what creates profit opportunities and causes instability in the global economy results from the failure of these assumptions. Herd behaviour, asset mispricing and grossly imperfect information have led us to where we are today.
There is not, and never will be, an economic theory of everything. Physics may, or may not, be different. But the knowledge we can hope to have in economics is piecemeal and provisional, and different theories will illuminate different but particular situations. We should observe empirical regularities and – as in other applied subjects such as medicine and engineering – we will often find pragmatic solutions that work even though our understanding of why they work is incomplete.
Max Planck, the physicist, said he had eschewed economics because it was too difficult. Planck, Keynes observed, could have mastered the corpus of mathematical economics in a few days – it might now have taken him a few weeks. Keynes went on to explain that economic understanding required an amalgam of logic and intuition and a wide knowledge of facts, most of which are not precise: "a requirement overwhelmingly difficult for those whose gift mainly consists in the power to imagine and pursue to their furthest points the implications and prior conditions of comparatively simple facts which are known with a high degree of precision". On this, as on much else, Keynes was right.