In no sphere of human activity is such a margin to be left for this intruder, because none is so much in constant contact with him on all sides.

He increases the uncertainty of every circumstance, and deranges the course of events.
From this uncertainty of all intelligence and suppositions, this continual interposition of chance, the actor in War constantly finds things different from his expectations; and this cannot fail to have an influence on his plans, or at least on the presumptions connected with these plans.

If this influence is so great as to render the pre-determined plan completely nugatory, then, as a rule, a new one must be substituted in its place; but at the moment the necessary data are often wanting for this, because in the course of action circumstances press for immediate decision, and allow no time to look about for fresh data, often not enough for mature consideration.
But it more often happens that the correction of one premise, and the knowledge of chance events which have arisen, are not sufficient to overthrow our plans completely, but only suffice to produce hesitation.
Our knowledge of circumstances has increased, but our uncertainty, instead of having diminished, has only increased.

The reason of this is, that we do not gain all our experience at once, but by degrees; thus our determinations continue to be assailed incessantly by fresh experience; and the mind, if we may use the expression, must always be "under arms." Now, if it is to get safely through this perpetual conflict with the unexpected, two qualities are indispensable: in the first place an intellect which, even in the midst of this intense obscurity, is not without some traces of inner light, which lead to the truth, and then the courage to follow this faint light.

The first is figuratively expressed by the French phrase coup d'oeil.