LONDON — "The ideas of economists and political philosophers, both when they are right and when they are wrong, are more powerful than commonly understood. Practical men, who believe themselves to be quite exempt from any intellectual influences, are usually the slaves of some defunct economist.''

So wrote John Maynard Keynes. I suspect, though, that a greater danger lies elsewhere — that practical men and women employed in the policymaking functions of central banks, regulatory agencies, governments and financial institutions' risk-management departments tend to gravitate to simplified versions of the dominant beliefs of economists who are, in fact, very much alive.

At least in the arena of financial economics, a vulgar version of equilibrium theory rose to dominance in the years before the financial crisis, portraying "market completion" as the cure to all problems, and mathematical sophistication decoupled from philosophical understanding as the key to effective risk management.