Maimonides (Rambam), in his Shemonah Perakim (“Eight Chapters”), the introduction to Pirkei Avot (Sayings of the Fathers), describes some fundamental ideas regarding the human soul and ethics. An important image he uses therein is that of the sick soul (nefesh cholah).
One of the questions within Jewish thought is this: If the yetzer hara (the inclination to evil) disappears in the time of Mashiach, what happens to our free will?