The thoughtful reader will note in this table the fact, that three of these elements are objective, that is, outward and visible; and the other three are subjective, that is, inward and invisible. Persons, Efforts, Renderings, are seen and known of all men; Desires, Estimates, Satisfactions, can be directly known only to the persons who feel and make them. This is a peculiarity of Political Economy, that has been far too little observed even when it has been observed at all. Objective and subjective elements in it meet and mingle in each transaction. Indeed, they alternate, as is shown in the table above: first a Seen, and then an Unseen, Element throughout. It is this commingling of outward and inward, visible and invisible, that makes all the difficulty and gives all the fascination in Political Economy. Whatever carries us into the steady though billowy play of universal human nature is at once difficult and fascinating. Quite contrary, however, to a common impression, the _certainty_ both of action and prediction in all the other Sciences as well as in Economics lies rather in the unseen elements than in those that are seen. Take for an example the calculation of an eclipse: it is not so much from what is visible in the heavens and on the earth that the astronomer infers and predicts to the instant the shadow of one orb thrown upon another, as it is from the wholly hidden but ever-enduring forces of gravitation constantly relating these orbs one to the other. So it is of the Sciences generally; progress is made in them and certainties are reached in connection with them, "_while we look not at the things which are seen, but at the things which are not seen; for the things which are seen are but for a time; but the things which are not seen are everlasting_." Invisible Desires and Satisfactions felt in connection with Exchanges are among the most constant elements of human nature; they, as it were, give birth to the relatively more transient (though visible) data of Efforts and Renderings; while inferences and conclusions and even predictions may be securely drawn from all of these, giving a solid ground for Political Economy to stand on,--almost as solid as the ground of the chief Physical Sciences.