Theories in metaphysics can only be tested by the _Reason_ of men, and reasonable men without apparent bias of motive take opposite views of Sensations and Intuitions and Volitions; while theories in economics, which can be even better tested by the _Reason_, have an additional and almost immediate and constantly recurring test through men's pockets and the tables of the Census. The people indeed sometimes deceive themselves, and are also too often deceived by others, in these matters of buying and selling; but it is none the less of the utmost consequence to this Science, that all the results of good and bad practice in Economics work themselves at last into a definite shape, into facts and figures that cannot lie. It is not, as in Ethics and Metaphysics, that tendencies and potencies only are ascertained, but everything speedily drifts into results measurable in numbers, which stand out like landmarks against the sky. It is just for this reason, as both the schools of the Roman lawyers admitted, namely, that we have in all cases the Return-Service as the outward expression and measure of the Desire and Effort of him who renders the service, and because it makes no difference which of two services exchanged be regarded as the return-service, that our Science is reared on the firm ground of objective realities, notwithstanding the strong subjective elements that have a constant part in it. (c) The third condition of a recognized Science is, that the logical processes appropriate to its class of facts have been already carefully applied to them and a certain number of "exact definitions and sound principles" have been already "educed from and applied to" them. We do not hesitate a moment to claim, that this condition also is fairly and fully met by Political Economy, and that this is a "Science" under the definition from every point of view, and particularly from this third point of view; and a few examples will now be given as a specimen merely of the logical work already achieved in Economics. First, Induction more or less busy for two thousand years has given at last an exact and acceptable definition of the Science, and impliedly an exact description of the class of facts with which it is conversant, namely, the Science of Sales, or what is exactly equivalent, the Science of Value; and Deduction at all points along this slow road has helped to correct and to broaden successive imperfect inductions, which an inquisitive and tentative and cautious spirit--the mainspring of Constructive Science--has instituted from time to time. Second, precisely the same processes often repeated have ascertained beyond question, that there are only three classes of Valuables and the exact differences between them, and that, consequently, only six cases of Value are possible to happen.