In this article, the author studies some central concepts in Avicenna’s and Tusı’s modal logics as presented in Avicenna’s Al-Isharat wa’l Tanbıhat (Pointers and Reminders) and in Tusı’s commentary. In this work, Avicenna introduces some remarkable distinctions in order to interpret Aristotle’s modal syllogistic in the Prior Analytics. The author outlines a new interpretation of absolute sentences as temporally indefinite sentences and argues on the basis of this that Avicenna seems to subscribe to the Principle of Plenitude. He also shows that he has no valid proof of the modal conversion rules and that he uses some rather ad hoc distinctions to show that Aristotle’s modal syllogistic is correct. The author also notes some interesting differences between Avicenna’s and Tusı’s approaches to modal logic.