Source 1 · Rishonim
VerifiedKuzari
Kuzari 5:20
In the fifth essay, section twenty, Rabbi Yehuda HaLevi discusses the three powers of the soul, describing the intellectual, the emotional, and the instinctual faculties, and their roles in spiritual and physical life.
וְהַטִּבְעִיִּים – מֵסִּבּוֹת אֶמְצָעִיּוֹת מוּכָנוֹת לָהֶם וּמַגִּיעוֹת אוֹתָם אֶל תַּכְלִית שְׁלֵמוּתָם, בְּעוֹד שֶׁלֹא יִמְנַע מוֹנֵעַ מֵאֶחָד מֵהַשְּׁלֹשָׁה חֲלָקִים.
This course is not compulsory, because the whole thing is potential, and the mind wavers between an opinion and its opposite, being permitted to turn where it chooses.