Chapter 9 · The Royal Secret · Krishna said
Bhagavad Gita 9.2 — King of knowledge
राजविद्या राजगुह्यं पवित्रमिदमुत्तमम्। प्रत्यक्षावगमं धर्म्यं सुसुखं कर्तुमव्ययम् ॥
rājavidyā rājaguhyaṃ pavitramidamuttamam | pratyakṣāvagamaṃ dharmyaṃ susukhaṃ kartumavyayam ||
Meaning
This is the king of knowledge and the king of secrets, supremely pure. It can be directly experienced, is in line with dharma, is joyful and easy to practise, and never fades.