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PONTIFICIAE ACADEMIAE SCIENTIARVM SCRIPTA VARIA - 2¢ 
we have to start with some bounded part of the world of reality 
which I may call « R» (in a diagram on the blackboard). This 
hounded part of reality consists of certain elements or elementary 
units which also, in my view, are finite in number. It is possible 
to make observation(s), one or more, on each or some of these 
elementary units. In this way we get a system of observations which 
[ may call « O » (shown in the blackboard) which has to be distin- 
guished from « R », the reality, but is based in some sense on this 
reality. With any given set of observations « O », it is possible to 
make a model, which I may call «M». Now, there are two gaps, 
which are important: the system of observations « O » may not be 
valid, that is, may not be adequately representative of « R » the 
world of reality. Secondly, a model « M » (or models) may not be 
relevant or adequate. At this stage, objectives or aims must be 
taken into consideration to examine whether any particular model 
« M » is relevant or adequate. For the Study Week the objective 
or aim is economic development; I am including the question of 
fluctuations within development. 
Model making, in the sense of Professor STONE, would belong to 
the world of reality, and not to the world of abstraction — if I have 
1nderstood him right, and must be, therefore, limited in time. 
Now, I shall pass on to a second point. In this Study Week we 
are interested in models in the world of reality. But models of what 
types? Coming from the under-developed areas, I suggest, ultimately 
not necessarily during this Study Week) the aim of economic deve- 
lopment must be that of the world as a whole. 
There is the system of the world as a whole, bounded, of course, 
in space and also in time, that is, time up to what is of interest to 
as, 1970 or 1980 or 2000 but perhaps not 5,000 A.D. If I take the 
system of the world as a whole, we have three broad areas: briefly: 
WEST and EAST (in the political sense) and DEVELOPING coun- 
tries. I think we have to think of the world system as acting and 
interacting between these three areas, rather than as a single unit. 
I shall use the symbols W, E and D (on the blackboard) without 
trying to define where is the exact geographical boundary of the 
-q 
Stone - pag. 88