How did medieval Jewish scholars, from Saadia Gaon to Rabbi Isaac Abarbanel, imagine a world that has experienced salvation? Is the Messianic reality identical to our current world, or is it a new world entirely? This work explores how a rationalist can remain calm in light of the seductive promises of the various apocalyptic teachings of Antiquity regarding the Messianic world. This book deals with the encounter between thinking based on pure reason, on the one hand, and the imagination seeking a vision of the future, on the other. The tension between a naturalistic approach and an apocalyptic approach to the history of the messianic idea--which is fundamental to this history of Jewish philosophy in the Middle Age--is surveyed here expansively, relying on dozens of print sources as well as manuscripts