PONTIFICIAE ACADEMIAE SCIENTIARVM SCRIPTA VARIA - 2! 
venient time without affecting the composition of the stock of 
assets at the beginning of the steady-state period. This has 
enabled me to speak of the assets needed at the beginning of 
1970 independently of the distribution of investment through 
the 1960’s, and thus to make a sharp distinction between the 
two models. In fact we know that the relationships I have 
assumed to be fixed do change with time, and therefore that 
we cannot calculate exactly the assets needed to provide a 
given capacity at the beginning of the steady-state period until 
we know the timing of investment through the transitional 
period. Here again, as in the case of consumption, the problem 
can only be solved by iterating between the two models. 
3. WHY A DUAL MODEL? 
Questions are sometimes raised about the method I have 
just outlined. Why, it is asked, do we need two models? 
Could we not work out a path which would carry consumption 
into some preassigned rate of growth as soon as possible? My 
answer is that at the present stage of the work it seems easier 
to divide the problem into two. With a single model it would 
be impossible to say in advance when the growth rate of the 
path calculated by the model would approximate to the growth 
rate preassigned for the steady state. In other words, it would 
be impossible to say how long the transitional period would 
last: the model would be a transitional model without any 
time limit. Consequently, it would be necessary to take a 
view on preferences, technology, and other variable factors, 
for an indefinite time-span in the future. 
Also, the desire for a single model is sometimes accom- 
panied by a belief that it must always be possible to move 
smoothly into a faster rate of growth without ever falling below 
the initial growth rate of the system. We may hope that this 
1] Stone. - pag. 32