We reconstruct as much as we can the part of al-Fārābī's treatment
of modal logic that is missing from the surviving pages of his Long Commentary on
the Prior Analytics. We use as a basis the quotations from this work in Ibn Sīnā,
Ibn Rushd and Maimonides, together with relevant material from al-Fārābī's
other writings. We present a case that al-Fārābī's treatment of the dictum de omni
had a decisive effect on the development and presentation of Ibn Sīnā's modal
logic. We give further evidence that the Harmonisation of the Opinions of Plato and
Aristotle was not written by al-Fārābī
