Kaṭhopaniṣad 2.11-12—Adhyātma-yoga

“O Naciketā, you, on becoming enlightened, have rejected them all by examining patiently the highest reach of desire, the support of the universe, the infinite results of meditation, the other shore of fearlessness, the extensive course of Hiraṇyagarbha that is praiseworthy and great, as also your own state. The intelligent man gives up happiness and sorrow by developing concentration of mind on the Self, and thereby meditating on the old Deity who is inscrutable, lodged inaccessibly, located in the intellect, and seated in the midst of misery.“