I have argued that forecasts will be based on more information than is contained in econometric models and in general on information differing from a… - Kenneth Arrow

The fundamental fact which causes the need for discussing public values at all is that all significant actions involve joint participation of many individuals. Even the apparently simplest act of individual decision involves the participation of a whole society.
It is important to note that this observation tells us all non-trivial actions are essentially the property of society as a whole, not of individuals. It is quite customary to think of each individual as being able to undertake actions on his own (e.g., decisions of consumption, produc- tion, and exchange, moving from place to place, forming and dissolving families). Formally, a social action is then taken to be the resultant of all individual actions. In other words, any social action is thought of as being factored into a sequence of individual actions.

There is no need to enlarge upon the importance of a realistic theory explaining how individuals choose among alternate courses of action when the consequences of their actions are incompletely known to them. It is no exaggeration to say that every choice made by human beings would meet this description if attention were paid to the ultimate implications.

Krugman's whole attack is directed at a statement made neither by Arthur nor by Cassidy. Krugman has not read Cassidy's piece with any care nor has he bothered to review what Arthur has in fact said.