638 PONTIFICIAE ACADEMIAE SCIENTIARVM SCRIPTA VARIA - 
FN 
namic problems, some difficulties have began to arise. For, 
some data — i.e., factors to be accepted as given from outside 
economic analysis — are in fact much more variable in time 
than some of the economic magnitudes taken as variables. In 
such circumstances, the traditional association has made the 
choice between variables and constants slowly undergo a serious 
distortion. Some magnitudes have been kept as variables al- 
though their variability is not important, and other magnitudes, 
whose variability is an essential feature of the long-run growth 
pattern, have been left out of the analysis altogether. 
It is precisely to avoid this distortion that the theoretical 
framework developed in the present work has been laid down, 
straight from the beginning, with reference to the requirements 
of a long-run dynamic analysis. The criterion for the choice 
of the hypotheses is a consequence of this approach. I have 
considered as typical variables those quantities which — inde- 
pendently of whether they are to be explained or not — un- 
dergo changes of an irreversible character as time goes on, 
by incessantly moving on in the same direction. Although the 
changes might be quite negligible within a single short period 
of time, each period marks for these quantities a step forward, 
in a slow but cumulative movement. On the other hand, I 
have assumed as constants those quantities which, in the long 
run, do not present any tendency (or for which there is no 
reason to expect any tendency) to move in any direction. 
These quantities, of course, may change quite a lot from 
one period to another, but the point is that their changes are 
temporary and reversible. Even if they do go for some time 
in a particular direction, they cannot go on indefinitely, and 
they are bound to come back to where they started. 
The difference between these two types of quantities can be 
immediately perceived if we consider a long period of time. 
Compare, for example, the American economy in 1860 and 
in 1960. It is quite possible and easy to claim that, within 
this period. magnitudes like the average time-life of the equip- 
"10] Pasinetti - pag. 68