On the sixth centenary commemoration (2006) of Ibn Khaldun, the focus in this essay is on the forgotten concept of 'human nature' in the studies of those who have dealt with his 'new social science' (ilmu al-umrani al-bashari). Though the concept is essential for the study of Ibn Khaldun's interdisciplinary thought; it has hardly been studied by those who have focused their scholarly work on the author of the Muqaddimmah's thought heritage. This essay attempts to shed light on Ibn Khaldun's concept of human nature. Here, three different types of the latter are identified in Ibn Khaldun's vision of humans. These types of human nature do not only affect the behaviours of single members of human societies, but they do also influence the rise and fall of the entire human societies and civilisations.