146 
POLITICAL ECONOMY 
want to make outstanding in the present 
chapter ; and there is involved in it the 
further idea that in the form which the 
theory of value assumes in the theory of 
international trade marginal incidents play 
parts as pre-eminent as those assigned to 
them in the economic scenes which we have 
already contemplated. At the same time, of 
course, I shall have to make perfectly plain 
the salient points in the doctrine of compara 
tive costs. 
The best way to realise the relation of part 
and whole which subsists between the two 
theories referred to, and at the same time 
to get an understanding of the principles 
of foreign trade, is to begin by dissecting the 
more complex of the two theories, that is 
the theory of home price. The theory of 
home price may be helpfully regarded as 
consisting of two parts, first, of a pure theory 
of exchange, or theory of barter, as it is 
sometimes called, and, secondly, of a theory 
of the reactions which follow upon the 
process of exchange. We shall at once 
analyse the former theory. 
It is premised that certain articles have 
been produced in certain ratios and that 
there exist given quantities of these articles 
to dispose of. The problem is to determine