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The Rambam explicitly acknowledges that there is something called "secrets of Torah." His words (or at least R' Kapach's translation of his words) are (Moreh Nevochim 2:33): שאי אפשר להכנס למעמד הר סיני יותר משיעור זה שהזכירו לפי שהוא מכלל סתרי תורה ... as it is impossible to enter into ...