SEMAINE D'ÉTUDE SUR LE ROLE DE L’ANALYSE ECONOMETRIOQOUE ETC. 
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STONF 
I should like to thank you all for the very interesting discussion 
on my paper and I am sure that you will forgive me if I do not 
attempt to reply individually but merely try to summarise the posi- 
tion. In 1927 the biologist J.B.S. HALDANE published a book called 
Possible Worlds in which he tried to examine how far certain things 
were possible from the standpoint of physical, chemical or biological 
laws. Could one imagine, for example, an elephant ten times larger 
than the elephants that are actually known? And, if not, what was 
it that prevented us from imagining that such animals could exist? 
My type of model building, and, I think, many other people’s model 
building too, is concerned with doing exactly the same sort of thing 
for an economic system: we should like to obtain certain results; 
could we imagine that they could come about? Now one may ask the 
question: how is one to decide such things? And this seems to me 
to get to the heart of the difference between predictive models and 
models which aim at projections based on hypotheses about changes 
that might be introduced into the world. If we assume that the 
world will continue to work as it has in the past, and if we do not 
like some of the consequences of this, we may ask: could we change 
things for the better? The answer is not necessarily « yes » but, 
equally, it is not necessarily « no », because the way in which the 
world in fact works is not by definition the best possible way. It 
seems to me that in trying to decide questions of this kind one must 
follow the principle that upholders of laissez-faire claim that laissez- 
faire maintains, namely the maximising principle: that wherever you 
see economic action taking place it is always successfully directed to 
maximising something that one wants to have maximised. Now if 
this were actually the case there would be very little need to build 
models because we should already have a perfect system which could 
not be improved. It can only be in the belief that this is not actually 
the case that we have interested ourselves in the rather exacting anc 
energetic pursuit of economic model building. 
Another question that came up and which I think is wotiu 
| Stone - pag. 109