A number of theories have been proposed regarding Avicenna's madhab (school of thought within Islamic jurisprudence). Medieval historian Ẓahīr al-dīn al-Bayhaqī (d. 1169) considered Avicenna to be a follower of the Brethren of Purity. On the other hand, Dimitri Gutas along with Aisha Khan and Jules J. Janssens demonstrated that Avicenna was a Sunni Hanafi. However, the 14th cenutry Shia faqih Nurullah Shushtari according to Seyyed Hossein Nasr, maintained that he was most likely a Twelver Shia. Conversely, Sharaf Khorasani, citing a rejection of an invitation of the Sunni Governor Sultan Mahmoud Ghazanavi by Avicenna to his court, believes that Avicenna was an Ismaili. Similar disagreements exist on the background of Avicenna's family, whereas some writers considered them Sunni, some more recent writers contested that they were Shia.
Who thought that Avicenna was a Brethren of Purity follower? (If the question is unanswerable, say "unanswerable")
Medieval historian Ẓahīr al-dīn al-Bayhaqī