Clausewitz has treated practically every chief branch of strategy and tactics (except, of course, the present-day developments of railways, telegraphs, quick-firing guns, smokeless powder, universal service armies, etc.). The whole of his bulky work "On War" is full of interesting and sometimes eloquent and almost poetical passages, of concentrated, pregnant, and far-reaching thoughts on every subject. Through all these it is, of course, impossible to follow him in any introduction. One can really do no more than urge all to read Clausewitz for themselves, to go to the fountain-head, to the master-work itself. In the short space to which I have restricted myself, I propose, therefore, to concentrate on a few of his leading ideas, reluctantly leaving out many others which are really almost just as good. THEORY AND PRACTICE One of the things for which we are most deeply indebted to Clausewitz is that he has shown us clearly the proper place of theory in relation to practice. "It should educate the mind of the future leader in war, or, rather, guide him in his _self-instruction_, but _not_ accompany him on to the battlefield; just as a sensible tutor forms and enlightens the opening mind of a youth without therefore keeping him in leading-strings all his life."[8] Again, "In real action most men are guided by the tact of judgment, which hits the object more or less accurately, according as they possess more or less genius. This is the way in which all great generals have acted, and therein partly lay their greatness and their genius, in that they always hit upon what was right by this tact. Thus also it will always be in _action_, and so far this tact is amply sufficient. But when it is a question not of acting one's self, but of convincing others _in consultation_, then all depends upon clear conceptions and demonstrations and the inherent relations; and so little progress has been made in this respect that most deliberations are merely a contention of words, resting on no firm basis, and ending either in every one retaining his own opinion, or in a compromise from mutual considerations of respect, a middle course really without any value. Clear ideas on these matters are not, therefore, wholly useless."[9] How true this is any one will admit who reflects for a moment upon the great diversity of opinions on almost every subject held in our army, just because of this want of a central theory common to all. In the domain of tactics it is evident that this holds good even as in strategy, for a common central theory of war will produce a more or less common way of looking at things, from which results more or less common action towards the attainment of the common object. REJECTION OF SET AND GEOMETRICAL THEORIES