Search facilities can help fake erudition.  So go to www.econlib.org > books > 
JS Mill, and search each book individually for the word "assumption".  Now you 
can use your human judgement and hey presto!!!  [Marx may have accused him of 
soulless syncretism (Nachwort to the 2nd edn of Das Kapital)but I love that 
prose]. 


Essays on Some Unsettled Questions of Political Economy 
Essay V. On the Definition of Political Economy. in paragraph V.46 

In the definition which we have attempted to frame of the science of Political 
Economy, we have characterized it as essentially an abstract science, and its 
method as the method a priori. Such is undoubtedly its character as it has been 
understood and taught by all its most distinguished teachers. It reasons, and, 
as we contend, must necessarily reason, from assumptions, not from facts. It is 
built upon hypotheses, strictly analogous to those which, under the name of 
definitions, are the foundation of the other abstract sciences. Geometry 
presupposes an arbitrary definition of a line, "that which has length but not 
breadth." Just in the same manner does Political Economy presuppose an 
arbitrary definition of man, as a being who invariably does that by which he 
may obtain the greatest amount of necessaries, conveniences, and luxuries, with 
the smallest quantity of labour and physical self-denial with which they can be 
obtained in the existing state of knowledge. It is true that this definition of 
man is not formally prefixed to any work on Political Economy, as the 
definition of a line is prefixed to Euclid's Elements; and in proportion as by 
being so prefixed it would be less in danger of being forgotten, we may see 
ground for regret that this is not done. It is proper that what is assumed in 
every particular case, should once for all be brought before the mind in its 
full extent, by being somewhere formally stated as a general maxim. Now, no one 
who is conversant with systematic treatises on Political Economy will question, 
that whenever a political economist has shown that, by acting in a particular 
manner, a labourer may obviously obtain higher wages, a capitalist larger 
profits, or a landlord higher rent, he concludes, as a matter of course, that 
they will certainly act in that manner. Political Economy, therefore, reasons 
from assumed premises�from premises which might be totally without foundation 
in fact, and which are not pretended to be universally in accordance with it. 
The conclusions of Political Economy, consequently, like those of geometry, are 
only true, as the common phrase is, in the abstract; that is, they are only 
true under certain suppositions, in which none but general causes�causes common 
to the whole class of cases under consideration�are taken into the account. 


Nicholas J. Theocarakis