752 
PONTIFICIAE ACADEMIAE SCIENTIARVM SCRIPTA VARIA - 28 
The Case of the Model 
Although it facilitates computation, this convention allocates 
a particular vole to a specified interval of time, the year, which 
is both arbitrary from a theoretical standpoint, and involves 
running a risk of suggesting erroneous ideas. 
It is for this reason that in the present study, investment 
is defined to include any outlay incorporated in a durable good, 
whatever its longevity. Under such a definition any output of 
primary factors which does not appear immediately as con- 
sumed income is considered as an investment. It is thus a 
simple matter to see the conceptual changes in the notions of 
G.N.P., gross investment and amortisation when the frame- 
work of the present theory is adopted. 
If we consider the concept of a continuous characteristic 
function 9, depreciation Adt is equal to the primarv income 
Redt which emerges in the national consumed income Rod? of 
instant # and we have 
(116-12) 
A=R, 
whence 
(116-13) 
P=R+A 
~R+R. 
using P to correspond to the equivalent in the present theory 
(in which any investment outlay, whatever its durability, is 
considered as an investment) of the conventional notion of 
G.N.P. (in which a minimum durability of one, or even of 
several years is taken as the boundary between operating ex- 
penses and investment outlays). 
[11] Allais - pag. 36