I want to know the answer to the following question:
Who thought that Avicenna was a Brethren of Purity follower?
All the background I'm provided with is that "Medieval historian Ẓahīr al-dīn al-Bayhaqī (d. 1169) considered Avicenna to be a follower of the Brethren of Purity.". Is that enough to answer the question?
Answer: yes