112 PONTIFICIAE ACADEMIAE SCIENTIARVM SCRIPTA VARIA - 28 
discussing can be summarised briefly under the term « disaggregat- 
lon ». It certainly is not my belief that the way to get better models 
and a more realistic representation of the economy is to start with 
a general model of the kind I described and then simply make it 
larger and larger and larger. The reason why this is not a good idea 
is, firstly, that it is impossible to get, in any group, sufficient infor- 
mation to build so large a model and, secondly, that it is quite 
unnecessary to do so. If you are interested in the operation of the 
chemical industry, even if you are only interested in a certain group 
of activities brought together in a single firm, you can set up a model, 
as large or larger than mine, to analyse the operations of that 
ndustry or firm. But it will be quite impossible for any single 
group of scientists to integrate into their own work models of this 
degree of detail. But why should they; if the industries concerned 
are willing to do the job themselves and are obviously very much 
setter at doing it? And this goes not only for the industrial side of 
‘he model. The same can be said about many of the activities of 
government. It is too much to expect that a group of economists 
who have the problem of a general model on their hands will also 
be able to build models of, say, the health service, the educational 
system and the defence system. The right way, I think, to get this 
sort of disaggregation is to have a series of sub-models (which, 
however, must be linked to the main one), to decentralize the building 
of these models and to put this work in the hands of people with the 
necessary specialized knowledge. 
finally, I should like to say something about another recurring 
:heme in this morning’s discussion: the question of iteration. It seems 
:o me that this is the fundamental principle on which all learning 
and all model-building is based. One has to start somewhere. One 
knows perfectly well that one’s prototype model will not be a very 
perfect tool, but the really important thing is that one should set it 
up, see how the parts of the system interact, and check how the 
relationships of the model work out in practice. We are bound to 
start with relatively simple ideas, we are bound to start with relati- 
vely inaccurate facts. We can try to find out how far the facts 
1" 
Stone - pag. 110