13.18 ज्योतिषाम् of lights, अपि even, तत् That, ज्योतिः Light, तमसः from darkness, परम् beyond, उच्यते is said (to be), ज्ञानम् knowledge, ज्ञेयम् that which is to be known, ज्ञानगम्यम् attainable by knowledge, हृदि in the heart, सर्वस्य of all, विष्ठितम् seated.Commentary: The Supreme Self illumines the intellect, the mind, the sun, moon, stars, fire and lightning. It is selfluminous, The sun does not shine there, nor do the moon and the stars, nor do these lightnings shine and much less this fire. When It shines, everything shines after It all these shine by Its Light. (Kathopanishad 5.15 also Svetasvataropanishad 6.14)Knowledge Such as humility. (Cf.XIII.7to11)The knowable As described in verses 12 to 7.The goal of knowledge, i.e., capable of being understood by wisdom.These three are installed in the heart (Buddhi) of every living being. Though the light of the sun shines in all objects, yet the suns light shines more brilliantly in all bright and clean objects such as a mirror. Even so, though Brahman is present in all objects, the intellect shines with special effulgence received from Brahman. (Cf.X.20XIII.3XVIII.61)