The modern movement of Islamic revival has been compared with earlier efforts of a similar nature: The "oscillat between periods of strict religious observance and others of devotional laxity" in Islamic history was striking enough for "the Arab historian, Ibn Khaldun to ponder its causes 600 years ago, and speculate that it could be "attributed ... to features of ecology and social organization peculiar to the Middle East", namely the tension between the easy living in the towns and the austere life in the desert.