PONTIFICIAE ACADEMIAE SCIENTIARVM SCRIPTA VARIA - 
added, since national economic systems are never isolated, 
foreign trade. The variables relevant to these concepts can, 
as everyone knows, be set out in a system of national accounts. 
These accounts have the merit of all accounts: their entries 
satisfy certain arithmetic and accounting identities. 
The entries in accounts are values: sums of money. If an 
entry relates to a product, whether final or intermediate, or to 
a primary input, it can be decomposed into a quantity and a 
price. Thus values, quantities and prices have their places in 
an accounting system. Consistency requires, therefore, that we 
set up at least the main variables in our model within an 
accounting framework. 
The technique of doing this is called social accounting and 
consists essentially in subdividing the national accounts. For 
example, in input-output analysis we subdivide the national 
production account so as to provide a separate account for each 
group of commodities that the economy produces. When we 
start doing this we find that different parts of the economy 
habitually use different systems of classification. Our system 
of social accounts should accommodate these different classi- 
fications and show how they are reconciled. In this way we 
can establish behavioural or technical relationships for different 
parts of the economy which reflect the categories habitually 
used by each part. For example, private consumers’ expen- 
diture is usually expressed in terms ot a classification of goods 
and services which follows the lines of a consumer’s shopping 
list; and public consumers’ expenditure is usually expressed 
in terms of various purposes, such as education, health and 
defence. Neither of these classifications has a one-to-one cor- 
respondence with the industrial classification of products, and 
these products, in turn, are not in one-to-one correspondence 
with the industries in which they are produced. Consequently, 
if we wish to be able to establish relationships for the compo- 
nents of each of these categories we must make sure that each 
1] Stone - pag. 8