(a) Every science must have to begin with a definite Class of facts, which lie in an easily circumscribable field, and which are not likely to be confounded with other facts of a differing nature. Economy has such a class of facts, that lie in such a field, and that cut themselves off by sharp lines from all other things. _Valuables_ is its class of things. It has nothing to do with any other class of things. Its field is Value, or Sales, or Exchanges. This field is perfectly definite. Sales are never confounded with gifts, and are never confounded with thefts. They have a distinctive character of their own. They have always been in the world, will always be in the world in ever-multiplying volume, and no one ever mistakes their main features for anything else. Anything whatsoever that is salable, or is about to be made so, comes within the view of Economics, and scientifically it cares for nothing else. While it finds its field definite, it also finds it broad. It has no wish to encroach on other sciences, nor will it tolerate any encroachments on its own. Before anything is sold, or is being made ready to sell, it cares nothing what other science employs itself upon that thing; after the thing is sold, Economy loses its interest in it, and other sciences may take it up if they choose. Valuableness is the one quality that constitutes the Class of things with which the Science is conversant, and it claims complete jurisdiction over all things just so far forth as they have this one quality, and no farther. Now there _is_ in the actual world such a Class of things; its exterior boundaries have been exactly ascertained by a long series of Inductions and Deductions, tentative, corrective, and confirmatory; and accordingly, Political Economy has now in full possession the first grand condition of a Science. (b) This great class of facts, thus reached by logical Generalization and grasped and held by a mental Conception and fixed by an adequate verbal Definition, is remarkably open to all the logical processes of reasoning, by which alone sciences are constructed, and thus possesses in full measure the second grand condition of the Sciences. Not one logical resource is denied to the economists: all the tools of the scientific workshop are at their hands. Let us now catalogue these in their order.