“Only one advantage still remained to Mandred, an edge that Vrrmik could never take from him. The soldiers who marched behind his banner were men and it was the hearts of men that beat within their breasts, hearts that could be moved to selflessness, could be stirred to valour and fired with courage. However numerous Vrrmik’s horde, they were skaven, they were cringing beasts driven by fear and greed, incapable of believing in anything more vital than their own skins. Terror and avarice were the forces that drove them on, but such things could only stretch so far, overcome only so much. Mandred’s troops could endure more than Vrrmik’s monsters. That was the one strength the skaven could never equal.”
Book:Wolf of Sigmar