No one who attempts to lay down propositions for the guidance of mankind, however perfect his scientific acquirements, can dispense with a practical knowledge of the actual modes in which the affairs of the world are carried on, and an extensive personal experience of the actual ideas, feelings, and intellectual and moral tendencies of his own country and of his own age. The true practical statesman is he who combines this experience with a profound knowledge of abstract political philosophy. Either acquirement, without the other, leaves him lame and impotent if he is sensible of the deficiency; renders him obstinate and presumptuous if, as is more probable, he is entirely unconscious of it. Such, then, are the respective offices and uses of the _à priori_ and the _à posteriori_ methods--the method of abstract science, and that of specific experiment--as well in Political Economy, as in all the other branches of social philosophy. Truth compels us to express our conviction that whether among those who have written on, these subjects, or among those for whose use they wrote, few can be pointed out who have allowed to each of these methods its just value, and systematically kept each to its proper objects and functions. One of the peculiarities of modern times, the separation of theory from practice--of the studies of the closet, from the outward business of the world--has given a wrong bias to the ideas and feelings both of the student and of the man of business. Each undervalues that part of the materials of thought with which he is not familiar. The one despises all comprehensive views, the other neglects details. The one draws his notion of the universe from the few objects with which his course of life has happened to render him familiar; the other having got demonstration on his side, and forgetting that it is only a demonstration _nisi_--a proof at all times liable to be set aside by the addition of a single new fact to the hypothesis --denies, instead of examining and sifting, the allegations which are opposed to him. For this he has considerable excuse in the worthlessness of the testimony on which the facts brought forward to invalidate the conclusions of theory usually rest. In these complex matters, men see with their preconceived opinions, not with their eyes: an interested or a passionate man's statistics are of little worth; and a year seldom passes without examples of the astounding falsehoods which large bodies of respectable men will back each other in publishing to the world as facts within their personal knowledge. It is not because a thing is _asserted_ to be true, but because in its nature it _may_ be true, that a sincere and patient inquirer will feel himself called upon to investigate it. He will use the assertions of opponents not as evidence, but indications leading to evidence; suggestions of the most proper course for his own inquiries.