A preface is perhaps a place to offer some explanation, and certainly a place to make acknowledgements and offer thanks.

The only part of this book which may, I think, require some explanation is Part II, and the explanation is not independent of some debts I wish to acknowledge. When I was an undergraduate at the London School of Economics the works of Oscar Lange and Abba Lerner were on the reading list, and, true to the liberal and open tradition of the School, “socialist economics” was prominent on the agenda. Later, when I was on the teaching staff of the School, I encountered the work of Karl Popper and some of his colleagues, as well as that of the great, if misguided, Bill Phillips. It was not until I had been for some years at this University that I realized that a feed-back, or iterative, control system might be designed in such a fashion as to avoid some of the difficulties inherent both in Phillips' control systems and in central planning. Such a system requires the prior selection of what I call a Criterion Function. Such a function must have the properties that it signals clearly, probably by reaching an extremum, that the target has been reached, and sufficient information to estimate its value must be generated during the iterative process itself.