In particular, he was known for his intensely negative views of Ibn Arabi, Hallaj, Ibn al-Farid, Ibn Sab'in and Shushtari, some of the primary figures in Sufism.
Other important philosophers and thinkers in the Sicilian Questions referred to are, in alphabetical order, Alexander of Aphrodisias, Anaxagoras, Berosus, Crates, Diogenes, Euclid, Al-Farabi, Galen, Al-Ghazali, Al-Hallaj, Ibn Bajja (Avempace) Ibn Sina (Avicenna), Iamblichus, Mellow, Parmenides, Pythagoras, Plato, Socrates, Themistius, Theophrastus, and Zeno of Elea.
The anonymous Dutch translator, "S.D.B.", gave a concise biographical review of the philosophers related to the text: Al Farabi, Avicenna, Al Ghazali, Ibn Bajjah, Ibn Rushd, Junayd, and Mansur Al-Hallaj (with a description of his death and a reference to his famous "Ana al-Haqq").
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By 1922, his introduction to the technical lexicon of Sufism and the Passion of Al-Hallaj initiated the first line of textual study, translation and publication of sources that developed into the watershed of which the chief engineers were Henry Corbin and Seyyed Hossein Nasr.