PONTIFICIAE ACADEMIAE SCIENTIARVM SCRIPTA VARIA - 28 
V1 
CONCLUSIONS 
I shall bring this paper to an end by summarising the phi- 
losophy of economic model-building which we are trying to 
follow in our work. 
First, it is useful at the outset to picture a model against 
the general background of knowledge, objectives and controls 
which would have to be taken into account if the model were 
ised for practical purposes. 
Second, the main use of a model is to help us in exploring 
possible worlds; in examining, that is to say, not only how a 
particular economic system works but how it might work, and 
n relating the performance of the component parts to the 
needs of the system as a whole. 
Third, a model must have coherence and realism. Coher- 
ence can be achieved by giving the model a suitable structure; 
realism is quite a different matter. To achieve realism we must 
use relationships which recognise the changing character of 
preferences and technology. Our initial efforts to do this may 
not give very accurate results, partly because of the difficulty 
of formulating relationships which are both realistic and man- 
ageable and partly because the factual information relating to 
‘he past is incomplete. But this does not mean that we should 
stay content with first approximations. 
Fourth, in choosing the relationships that are needed, such 
as consumption functions, production functions and so on, 
accepted economic theory provides a valuable guide. In for- 
mulating these relationships, however, economic theory is much 
less useful because most of its elaborated parts are based on 
a narrow, static view of the world. Within this narrow view, 
1] Stone - pag. 80