John Stuart Mill, On the Definition of Political Economy; and on the Method of Investigation Proper To It.
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 à 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.