Here we must go slowly, if we would go surely. We must first find out exactly what a Science is in general, and then ascertain in particular whether Political Economy bears all the marks and stands all the tests of the other genuine Sciences. What is a Science? _A Science is the body of exact definitions and sound principles educed from and applied to a single class of facts or phenomena._ The very first condition, accordingly, of any science is, that there be a single class of facts, objective or subjective, that can be separated from all other classes of facts, in the mind by a generalization and in words by a definition, and that such generalization and definition be clearly made and held; the second condition is, that the class of facts so circumscribed and defined be open to some or all of the logical processes of construction, of which the most important are Induction and Deduction; the third condition is, that the subordinate definitions and working principles within the inchoate Science be all educed from and applied to these circumscribed facts in strict accordance with these well-known logical processes; and the last condition is, that these definitions and principles have gradually become "_a body_," in which there is an organic arrangement of parts, all being placed in a just order and mutual interdependence. There is no old Science, and there can be no new Science, in which these four conditions do not meet and become blended; and the beauty of it is, that this Definition applies to any Science in all stages of its growth. No one of all the Sciences is as yet completed; but just so soon as any correct definitions and principles are drawn from and applied to any _class_ of things clearly circumscribed as such, and these definitions and principles are orderly arranged in a _body_, there is an incipient Science; and its progress towards perfection will proceed in precisely the same manner in which its foundations have been laid; new facts and principles and definitions will gradually be discovered, and these when reapplied to the class of things out of which they have sprung, will lead to corrections and adjustments and enlargements of the Science; and no matter how far these logical processes may be carried, the general Definition with which we start will also be found ample at the end of the journey. All of the Sciences without exception have been developed into their present position in just this manner; and they fall easily into three great classes, namely, the Exact, the Physical, and the Moral Sciences. The ground of this triple classification is partly the distinct subject-matter in the three classes of Sciences, and partly the distinctive prominence of one or more of the logical processes of construction in each.