Military history experts have not agreed on a precise definition of "morale". Clausewitz's comments on the subject have been described as "deliberately vague" by modern scholars. George Francis Robert Henderson, a widely read military author of the pre-World War I era, viewed morale as related to the instinct of self-preservation, the suppression of which he said was "the moral fear of turning back", in other words, that a willingness to fight was bolstered by a strong sense of duty. Henderson wrote: