In plain words
These Upaniṣads too divided it into three parts, the three yogas, and went on teaching men in the path of the three yogas. 'Yoga' means the feeling of offering. Whatever karma we do, we must perform it in the feeling of offering. Because it is practised in a feeling offered to God, it took the name daivārpita-bhāva. It followed the path of 'Sarvakarma bhagavatprītyartham'. Next, the second is upāsanā-yoga. Among the three yogas, upāsanā-yoga is to love God with the whole heart, with Prema, in purity of thought, word and deed. To love only for the fulfilment of a wish, for success in some task, for the fulfilment of worldly delusions, is not Prema. We must love for the sake of Prema. That is upāsanā-yoga. Next, the third is Jñāna-yoga. 'Sarvaṃ viṣṇumayaṃ jagat,' it declares: it is the very form of God. He who dwells in all is God alone. The feeling of the one Ātma is Jñāna, they said. Advaita-jñāna alone is true Jñāna, they said. To believe that divinity is present in all as the Ātma itself: that is Jñāna.
Students may have some doubts. When the forms are different, the names are different and their doctrines are different, you may doubt that such a feeling of oneness in all is possible at all. In the fathomless ocean countless waves keep rising. Each wave takes on a form of its own. It seems as if one wave has no connection at all with another. But in all the waves the water is one. None of the waves is in any way apart from the ocean. All these waves are in no way different from the ocean. In just the same way, though forms and names, doctrines and rites differ, in the ocean of Sat-Cit-Ānanda all these are like waves. So in the ocean of Sat-Cit-Ānanda everyone is like a wave.
In every man Sat-Cit-Ānanda lies hidden within. Then from the Upaniṣads the Bhagavad Gītā emerged. It taught the world in the way called the ṣaṭkas. It also took on a form called the ṣaḍaṅgas. There are eighteen chapters in the Gītā. Six chapters were given the name ṣaṭka. Three ṣaṭkas together complete the Gītā in eighteen chapters.