Acquiring Coup d’oeil, the General’s Discerning Glance
Among [a general’s] skills, the one by which the eighteenth century [C.E.] set the greatest store was that of coup d’oeil, a facility which enabled a commander to grasp the essentials of a situation and make a speedy and appropriate decision [in a glance].
The process of acquiring coup d’oeil began in peacetime, while the officer was out walking, riding, or hunting. One of the fundamental exercises was to fix a particular measurement in your mind, and the apply it over successively greater distances. The ordinary human pace was assumed to be about 2 feet, and the Prince de Ligne discovered that 80 such paces approximated to the maximum range at which he would consider shooting a hare. Three 80-pace units in turn yielded the length of an Austrian battalion, which came to 240 paces, including the 6 paces allowed for the battalion artillery. The estimation of numbers also demanded practice:
When you see laborers or a herd of cattle in a field, you should guess their number from a distance, then approach more closely and count them, so as to find your margin of error. By repeating this exercise over and over again you acquire a certain assurance of judgment, which will enable you afterwards to make an accurate assessment of a force of infantry or cavalry.
Eventually it became possible to envisage the most peaceful landscape in military terms….
— The Military Experience in the Age of Reason, pp. 140-41