PONTIFICIAE ACADEMIAE SCIENTIARVM SCRIPTA VARIA - 28 
DIAGRAM . 
: Model in its Environment 
> Experience 
the system would behave if left to itself but also what would be 
the problems and results of trying to change it in various res- 
pects, the model must be readily computable. Model-building 
is a scientific activity but I do not think that a usable model 
of an economic system can be built by a group of scientists 
working in isolation, for the simple reason that, however well- 
endowed the group may be, it cannot possibly know as much 
as it needs to know about all aspects of the economy without 
the cooperation of many people engaged in a variety of practical 
tasks. The model-builders can of course build a prototype, but 
they will be very unwise if they do not seek a great deal of 
practical advice before they go fully into production. 
The combination of a model with a set of objectives gives 
rise to a policy. This is where the politician comes in: he for- 
mulates the objectives. He may specify objectives in detail or 
he may delegate his responsibility, as when, with whatever safe- 
1] Stone - pag. 4