Nahmanides, Sermon on Ecclesiastes

Jewish Mysticism
With respect to these matters and others like them one cannot understand their truth from one’s own mind (mida’at atzmo) but only through tradition (be kabbalah). This matter is explained in the Torah to whoever has heard the rationale for the commandments through a tradition (ta’am ha mitzvot be kabbalah) as is fitting. This refers to one who has received from a mouth which has received, going back to Moses, our teacher, [who received] from God.

Translated by Elliot Wolfson in- “Beautiful Maiden Without Eyes- Peshat and Sod in Zoharic Hermeneutics.” In The Midrashic imagination – Jewish exegesis, thought, and history. Edited by Michael Fishbane, Albany – State University of New York Press, 1993, pp. 155-203, from Kitvei Ramban, edited by Charles Chavel, Jerusalem, Mosad ha-Rav Kook, vol. 1, p. 190