Nasir al-Din Tusi (d. 672/1274) was an influential philosopher, theologian, mathematician and astronomer, besides being the first director of the famous observatory at Maraghah near Tabriz as well as a man of politics. Author of a large number of scholarly works, he is especially famous for such treatises as his Tajrid al-i?tiqad on theology, the Zij-i Ilkhani on astronomy, and his Akhlaq-i Nasiri on ethics. The present volume contains a facsimile edition of an ancient copy of another famous work by him, the Hall mushkilat al-Isharat, which is his influential commentary on Avicenna's (d. 428/1037) groundbreaking Kitab al-isharat wal-tanbihat. The Isharat is commonly regarded as Avicenna's final statement on all there is to know in logic and philosophy. Directed at a restricted readership of trusted specialists, it was deliberately written in a terse, impenetrable style. From the many commentaries that were written on it, the one by Tusi would be decisive for the later Avicennan tradition.