"It should educate the mind of the future leader in war" is what Clausewitz demands from a useful theory; but he most expressly and unreservedly rejects every attempt at a method "by which definite plans for wars or campaigns are to be given out all ready made as if from a machine."[10] He mocks at Bülow's including at first in the one term "base" all sorts of things, like the supply of the army, its reinforcements and equipments, the security of its communications with the home country, and lastly the security of its line of retreat, and then fixing the extent of the base, and finally fixing an angle for the extent of that base: "And all this was done merely to obtain a pure geometrical result utterly useless" (Von Caemmerer). For the same reason Jomini's principle of the Inner Line does not satisfy him, owing to its mere geometrical nature, although he right willingly acknowledges "that it rests on a sound foundation, on the truth that the combat is the only effectual means in war" (Von Caemmerer). All such attempts at theory seem to him therefore perfectly useless, "because they strive to work with fixed quantities, while in war everything is _uncertain_, and all considerations must reckon with all kinds of variable quantities; because they only consider _material_ objects, while every action in war is saturated with _moral_ forces and effects; lastly, because they deal only with the action of _one_ party, while war is a constant reciprocal effect of _both_ parties" (Von Caemmerer). "Pity the warrior," says Clausewitz, "who is contented to crawl about in this beggardom of rules." "Pity the theory which sets itself in opposition to the mind"[11] (note, the moral forces). A THEORY TO BE PRACTICALLY USEFUL Clausewitz insists that a useful theory cannot be more than a thorough knowledge of military history and "reflection upon all the situations in which we can be placed in war." "What genius does must be just the best of all rules, and theory cannot do better than to show just how and why it is so." "It is an analytical investigation of the subject which leads to 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. 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."[12] KNOWLEDGE MUST BE THOROUGH