The first of these methods is a method of induction, merely; the last a mixed method of induction and ratiocination. The first may be called the method _à posteriori;_ the latter, the method _à priori_. We are aware that this last expression is sometimes used to characterize a supposed mode of philosophizing, which does not profess to be founded upon experience at all. But we are not acquainted with any mode of philosophizing, on political subjects at least, to which such a description is fairly applicable. By the method _à posteriori_ we mean that which requires, as the basis of its conclusions, not experience merely, but specific experience. By the method _à priori_ we mean (what has commonly been meant) reasoning from an assumed hypothesis; which is not a practice confined to mathematics, but is of the essence of all science which admits of general reasoning at all. To verify the hypothesis itself _à posteriori_, that is, to examine whether the facts of any actual case are in accordance with it, is no part of the business of science at all, but of the _application_ of science. 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,