The procedure involved in formulating such a detailed plan of action has been described previously in general terms (Chapter V). The method of determining the salient features of the operations required has also been discussed (in Section III of Chapter IV). Therefore, these matters are not repeated at this point. The problems distinctive of this procedure (the second step, as described in Chapter V) are subsidiary problems, in the sense that the incentive for their solution arises by reason of a decision already made by the commander, i.e., the basic Decision, and because they are problems which the commander recognizes are to be solved by himself and not by his subordinates. Assumptions. The commander's plan has been derived from an estimate of the situation based on the best information available to him. Complete and accurate information is frequently lacking; hence, many military plans consider contingencies which, to make a plan possible, have been accepted in the estimate as assumptions. The word assumption, when used to denote a basis for a plan, signifies "the taking of something for granted". It does not mean a conjecture, guess, or probability. The proposed action, resulting from a decision made under an assumption, is designed to be taken only upon the disclosure of the truth of the assumption. The fact that the assumption upon which the plan is based may prove false indicates the advisability of developing several plans based upon various sets of assumptions. It would be erroneous to believe that all contingencies can be foreseen, and to be content with a particular set of plans, all of which may prove to be wrong. It is not to be expected that a plan based upon assumptions will, in all respects, be suitable for use in an actual situation. For example, it will seldom occur that an elaborate Battle Plan, based upon assumptions as to the various types, dispositions, and strengths of forces present, the weather conditions, and the intent of the enemy, can be used without changes. On the other hand, a plan for the sortie of a fleet from a harbor under assumptions that high visibility exists, that airplanes can operate, and that hostile submarines will be the only force in opposition, may frequently be found entirely applicable to the actual situation, or so nearly so as to require only slight modification. It is possible so to standardize such plans that only minor variables need be indicated when the plan is to be used. The visualization of valid and useful assumptions frequently makes the most serious demands on professional knowledge and judgment.