of some Doctrines of Political Economy. 199 



made one of the foundations of the doctrines which have been 

 maintained. 



Axiom 3. If from aui/ cause the value of tJie produce of 

 a given quantity of land increases, new land will he cultivated, 

 or new capital will be employed on land already cultivated: 

 and this will he done till there is some land cultivated, or some 

 capital agriculturally employed, which returns no more than the 

 usual profits of capital without any surplus. 



A soil which is in the condition described in the last clause 

 pays no rent, and being in its quality the limit of the soils which 

 can be cultivated, it may be called the limiting soil; including 

 in this term also the last portion or dose of capital employed on 

 land previously cultivated ; viz. that portion which produces only 

 common profits. 



It is convenient for our calculations to assume that there is 

 always such a limiting soil and limiting dose of capital. It is 

 however not self-evident that this supposition is generally exact. 



The supposition would be true, if there existed an indefinite 

 number of kinds of land, differing by insensible shades of pro- 

 ductiveness; or an indefinite number of ways of employing suc- 

 cessive additions of capital, gradually diminishing in productiveness. 

 The assumption of such a series of soils seems to be somewhat 

 arbitrary : and the supposition of such a list of known ways of 

 employing additional capital on the same soil, seems to be not 

 at all obvious in theory, nor confirmed by the history of the 

 progress of agriculture. 



9. It is also necessary to premise some principles on the 

 subject of prices, as influenced by demand and supply. Supply 

 is of itself a quantity, and offers us no difficulty in measuring it 

 theoretically : but demand is of a more untangible and fugitive 

 nature. It consists originally of moral elements as well as physical : 



