Especially important is it that the men know how to march and how to care for themselves in the field. However well instructed a soldier may be he is of no use if at the time of battle he is back in the hospital. [Sidenote: Desirable instruction] Under the second heading, of things that it is desirable the men should know, are subjects which are essential for the officers to know but which are not equally essential for the men. Nevertheless it is an advantage to have them know as much as possible, provided the merely desirable instruction does not interfere with the proper training in essentials. Among these subjects are topography and the construction of temporary bridges; the list might be extended almost indefinitely. [Sidenote: Things to be understood and things to be made fixed habits] The instructor must further distinguish between the essential things which the men need merely to know or to understand and those which need to be practiced until they become habits. Those things the men will only be required to do off the battlefield, where they will have time to think and be in condition to use their heads, need only be known. Psychology teaches us that under great stress of danger and excitement a man can be depended upon to do only those things which have become fixed habits, and further, that under these same trying conditions, a man who has acquired by practice a habit of doing something a certain way cannot do that thing differently. Action contrary to habit requires thought, and mental activity is difficult if not impossible under the circumstances. Acting according to habit is merely following the line of least resistance. It is difficult to conceive of greater stress of danger and excitement than exists in a modern battle. Certainly there is no other case in which the knowledge of this psychological truth can be used to greater advantage than in training for battle. [Sidenote: Fixed habits in battle] As far as possible, then, all those things which the men must do under fire should be practiced until they become fixed habits. It has been said that if in the heat of battle a man even raises his rifle to his shoulder, before firing, it shows fair discipline. Not only must bringing the rifle to the shoulder be made a habit, but correct aiming and trigger pull whenever the rifle is brought to the shoulder must be made a habit, and one so strongly developed that these acts will always be done mechanically and without mental effort. [Sidenote: Habits of correct aiming and firing]