PONTIFICIAE ACADEMIAE SCIENTIARVM SCRIPTA VARIA - 28 
is planning for. With this in mind, we have divided our model 
into two parts, a long-run model and a short-run model, and 
a solution comes from iterating between the two. The purpose 
of the long-run model is to help in choosing a direction and 
a rate of steady expansion to be achieved from 1970 onwards; 
and the purpose of the short-run model is to choose a path to 
this goal. If the model were used for policy-making, it would 
of course be necessary to take a new view of long-run objectives 
at regular intervals. 
Ninth, an economic model should cover all aspects of eco- 
nomic activity. As it stands at present, our model is restricted 
mainly to the real side of the national economy, but we are 
now beginning to extend it into the financial sphere and hope 
eventually to develop it in other directions as well. 
Finally, we have started with a purely economic model, 
conceived on traditional lines, because there we felt on reason- 
ably firm ground. But we believe that the main motive forces 
of economic growth are to be found in human abilities and 
attitudes: organising capacity, acceptance of education and 
training, response to innovation, labour mobility, and so on. 
However, we could hardly have begun with these indefinite and 
on the whole badly documented areas of interest; and in any 
case it would have been useless to do so until we could embody 
them in a coherent picture of the socio-economic system. So, 
naturally enough, we decided to build out from the familiar 
and to use our working experience as the starting point for 
our work. 
Pythagoras’ remark, dgyn 8e tot fjpov mavtog, ‘the begin- 
aing is half the whole’, applies to social and economic model- 
building as it does to all complicated human endeavours. The 
only practical course is to build a prototype and then improve 
on it in the light of experience and needs. It is fairly safe to 
say that the modern aeroplane would never have come into 
being if no aeroplanes had been flown until they were as good 
as they are today. 
'1] Stone - pag. 82