To reason exactly, the student first must rid his mind of all preconceived notions; he must regard the matter under consideration as a blank sheet of paper, upon which nothing is to be written save those things which by the processes of logic and demonstration, are established as facts. There is much difference between the Art of Logic and mere conjecture. The calculations of arithmeticians, though rigorous and exact, are never difficult; because they relate to known quantities and to the palpable objects of nature. But when it is required to argue from combining circumstances, the least ignorance of uncertain and obscure facts breaks the chain and we are deceived every moment. This is no defect of the understanding, but error arising from plausible ideas, which wear the face of and are too quickly accepted for truth. A long chapter can be written on the different ways in which men lose themselves in their conjectures. Innumerable examples of this are not wanting, and all because they have suffered themselves to be hurried away and thus to be precipitate in drawing their conclusions. The part that the General, whether in Chess-play or in Warfare, has to act, always is more difficult because he must not permit himself the least mistake, but is bound to behave with prudence and sagacity throughout a long series of intricate processes. A single false deduction, or a movement of the enemy unintelligible to a commander, may lead him to commit an irremediable error; and in cases wherein the situation is beyond comprehension, his ignorance is invincible. For however extensive the human mind may be, it never is sufficiently so to penetrate those minute combinations necessary to be developed in order to foresee and regulate events, the sequence, utility and even existence of which, depend upon future contingencies. Incidents which are past, can be explained clearly, because the reasons therefor are manifest. But men easily deceive themselves concerning the future, which, by a veil of innumerable and impenetrable secondary causes, is concealed from the most prying inspection. In such situations, how puerile are the projects even of the greatest Strategist. To him, as much as to the tyro, is the future hidden; he knows not what shall happen, even on the next move. How then may he foresee those situations which secondary causes later may produce? Circumstances most often oblige him to act contrary to his wishes; and in the flux and reflux of fortune, it is the part of prudence to conform to system and to act with consistency. It is impossible to foresee all events. * * * * * “It is not possible,” writes the Count de Saxe, “to establish a system without first being acquainted with the _principles_ that must necessarily support it.” In corroboration of this is the opinion of Frederic the Great: