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.

Alternative Plans. The word "alternative" is generally applied to
contingent plans intended to accomplish a common task, but developed
from varying sets of assumptions. "A choice between several" is the
meaning of the word as here used. When such choice becomes necessary
in a situation not yet clarified, that plan will be selected which has
been derived from the set of assumptions considered by the commander
as most likely to be correct. The selected plan is usually called the
plan or the "accepted plan", and the other plans, coming from other
less likely but still possible sets of assumptions, are called
Alternative Plan No. 1, Alternative Plan No. 2, etc.

Naval tactical situations particularly lend themselves to the drawing
up of alternative plans in advance. There