Further, according to a division of the subject in _agreement with its nature_, the difficulties are not everywhere the same, but diminish the more results manifest themselves in the material world, and increase the more they pass into the moral, and become motives which influence the will. Therefore it is easier to determine, by theoretical rules, the order and conduct of a battle, than the use to be made of the battle itself. Yonder physical weapons clash with each other, and although mind is not wanting therein, matter must have its rights. But in the effects to be produced by battles when the material results become motives, we have only to do with the moral nature. In a word, it is easier to make a theory for _tactics_ than for _strategy_. 27. THEORY MUST BE OF THE NATURE OF OBSERVATIONS NOT OF DOCTRINE. The second opening for the possibility of a theory lies in the point of view that it does not necessarily require to be a _direction_ for action. As a general rule, whenever an _activity_ is for the most part occupied with the same objects over and over again, with the same ends and means, although there may be trifling alterations and a corresponding number of varieties of combination, such things are capable of becoming a subject of study for the reasoning faculties. But such study is just the most essential part of every _theory_, and has a peculiar title to that name. It is an analytical investigation of the subject that leads to an exact knowledge; and if brought to bear on the results of experience, which in our case would be military history, to a thorough familiarity with it. The nearer theory attains the latter object, so much the more it passes over from the objective form of knowledge into the subjective one of skill in action; and so much the more, therefore, it will prove itself effective when circumstances allow of no other decision but that of personal talents; it will show its effects in that talent itself. If theory investigates the subjects which constitute War; if it separates more distinctly that which at first sight seems amalgamated; if it explains fully the properties of the means; if it shows their probable effects; if it makes evident the nature of objects; if it brings to bear all over the field of War the light of essentially critical investigation—then it has fulfilled the chief duties of its province. It becomes then a guide to him who wishes to make himself acquainted with War from books; it lights up the whole road for him, facilitates his progress, educates his judgment, and shields him from error.