The stock market has forecast nine of the last five recessions. - Paul Samuelson

As old problems are conquered, we expect to turn up new problems. A discipline lives on its unsolvved problems; and so, for better or worse, economics is likely to be a lively subject for as many years ahead as man can see.

I reproach myself for a gross error. But I would reproach myself more if I had persisted in an error after observations revealed it clearly to be that. I made a deal of money in the late 1940s on the bull side, ignoring Satchel Paige’s advice to Lot’s wife, “Never look back.” Rather I would advocate Samuelson’s Law: “Always look back. You may learn something from your residuals. Usually one’s forecasts are not so good as one remembers them; the difference may be instructive.” The dictum “If you must forecast, forecast often,” is neither a joke nor a confession of impotence. It is a recognition of the primacy of brute fact over pretty theory. That part of the future that cannot be related to the present’s past is precisely what science cannot hope to capture. Fortunately, there is plenty of work for science to do, plenty of scientific tasks not yet done.

A later writer, such as Leijonhufvud, I knew to have it wrong, when he later argued the merits of Keynes’s subtle intuitions and downplayed the various (identical!) mathematical versions of The General Theory. The so-called 1937 Hicks or later Hicks–Hansen IS–LM diagram will do as an example for the debate.