In volume two of "Feudal Society", Marc Bloch writes in the chapter about nobility, under the section named "THE PROFESSION OF ARMS", that:
"the possession of manors was the mark of a genuinely noble status and, along with treasure in money or jewels, the only form of wealth which seemed compatible with high rank, this was due in the first place to the authority over other men which it implied. (Could there ever be a surer basis of prestige than to be able to say: ‘It is my will’?) But another reason was that the very vocation of the noble prevented him from engaging in any direct economic activity. He was committed body and soul to his particular function—that of the warrior. This fact, which is of fundamental importance, explains the rôle of the military vassals in the formation of medieval aristocracy."
It is not so clear to me the part i emphasized in bold-face. I think he is trying here to suggest that nobles had a vocation for war that prevented them from working directly. But I think I did not get the point: why do nobles have this vocation in the first place and what is it that justify this point?