Nor do all schools of thought within Islam admit the usefulness or legitimacy of philosophical inquiry. Some argue that there is no indication that the limited knowledge and experience of humans can lead to truth. It is also important to observe that, while "reason" ("'aql") is sometimes recognised as a source of Islamic law, this may have a totally different meaning from "reason" in philosophy. The historiography of Islamic philosophy is marked by disputes as to how the subject should be properly interpreted. Some of the key issues involve the comparative importance of eastern intellectuals such as Ibn Sina (Avicenna) and of western thinkers such as Ibn Rushd, and also whether Islamic philosophy can be read at face value or should be interpreted in an esoteric fashion. Supporters of the latter thesis, like Leo Strauss, maintain that Islamic philosophers wrote so as to conceal their true meaning in order to avoid religious persecution, but scholars such as Oliver Leaman disagree. The main sources of classical or early Islamic philosophy are the religion of Islam itself (especially ideas derived and interpreted from the Quran) and Greek philosophy which the early Muslims inherited as a result of conquests, along with pre-Islamic Indian philosophy and Persian philosophy. Many of the early philosophical debates centered around reconciling religion and reason, the latter exemplified by Greek philosophy. In early Islamic thought, which refers to philosophy during the "Islamic Golden Age", traditionally dated between the 8th and 12th centuries, two main currents may be distinguished. The first is Kalam, which mainly dealt with Islamic theological questions, and the other is Falsafa, which was founded on interpretations of Aristotelianism and Neoplatonism. There were attempts by later philosopher-theologians at harmonizing both trends, notably by Ibn Sina (Avicenna) who founded the school of Avicennism, Ibn Rushd (Averroes) who founded the school of Averroism, and others such as Ibn al-Haytham (Alhazen) and Abū Rayhān al-Bīrūnī. "ʿIlm al-Kalām" () is the philosophy that seeks Islamic theological principles through dialectic. In Arabic, the word literally means "speech". One of the first debates was that between partisans of the "Qadar" ( meaning "Fate"), who affirmed free will; and the "Jabarites" ( meaning "force", "constraint"), who believed in fatalism. At the 2nd century of the Hijra, a new movement arose in the theological school of Basra, Iraq.