Ibn Khaldun wanted to discover the underlying causes of this change. He was probably the last great Spanish Faylasuf; his great innovation was to apply the principles of philosophic rationalism to he study of history, hitherto considered to be beneath the notice of a philosopher, because it dealt only with transient, fleeting events instead of eternal truths. But Ibn Khaldun believed that, beneath the flux of historical incidents, universal laws governed the fortunes of society. He decided that it was a strong sense of group solidarity (asibiyyah) that enabled a people to survive and, if conditions were right, to subjugate others. This conquest meant that the dominant group could absorb the resources of the subject peoples, develop a culture and a complex urban life. But as the ruling class became accustomed to a luxurious lifestyle, complacency set in and they began to lose their vigour. They no longer took sufficient heed of their subjects, there was jealousy and infighting and the economy would begin to decline. Thus the state became vulnerable to a new tribal or nomadic group, which was in the first flush of its own asibiyyah, and the whole cycle began again.