Verse
राजविद्या राजगुह्यं पवित्रमिदमुत्तमम् ।प्रत्यक्षावगमं धर्म्यं सुसुखं कर्तुमव्ययम् ॥
rājavidyā rājaguhyaṃ pavitramidamuttamampratyakṣāvagamaṃ dharmyaṃ susukhaṃ kartumavyayam
This is the king-knowledge, the king-secret (the wisdom of all wisdoms, the secret of all secrets), it is a pure and supreme light which one can verify by direct spiritual experience, it is the right and just knowledge, the very law of being. It is easy to practise and is imperishable.
Gītā 9·2