Could you give me a definition of the term Usool al-Deen? If possible I would also know which theologians said what on Usool al-Deen.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

The scholars defined 'Ilm Usool Ad-Deen (the science of the principles of religion) as 'A science which enables a person to prove religious beliefs by stating appropriate evidences and by warding off misconceptions from it.'

Among the scholars who discussed this terminology in details is Siddeeq Ibn Hasan Al-Qanooji, in his book Encyclopaedia of Abjad Al-'Uloom, as he addressed this science and the stages that it went through, and mentioned the books that were published about it. He said: 'There are many books about this science, the best of which are the books of the scholars of Hadeeth (i.e. the science of Prophetic narrations) in which they proved the belief according to evidences from the Book of Allaah and the Sunnah of the Prophet and they refuted the views of the theologians who criticized it. Among these books are the ones of Shaykh Al-Islam Ibn Taymiyyah and his student Ibn Al-Qayyim.'

Siddeeq Ibn Hasan Al-Qanooji also discussed the names of this science and its benefits and so forth. 

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