From a linguistic point of view, these treatises with their appendages, the more mystic and recondite Aranyakas and the speculative Upanishads, have to be considered as forming the connecting link between the Vedic and the classical Sanskrit.
YOGI, a Hindu religious ascetic. The word yoga means union, and first occurs in the later Upanishads; and yogi means one who practises yoga, with the object of uniting his soul with the divine spirit.
Impressed with the perversions and corruptions of popular Hinduism, Ram Mohan Roy investigated the Hindu Shastras, the Koran and the Bible, repudiated the polytheistic worship of the Shastras as false, and inculcated the reformed principles of monotheism as found in the ancient Upanishads of the Vedas.
So great were his learning and piety that he was regarded as an incarnation of Siva, and his works (commentaries on the Vedanta Sutras, the Bhagavad Gita and the Upanishads) exercised a permanent influence on Hindu thought.
But the Buddha, while rejecting the sacrifices and the ritualistic magic of the brahmin schools, the animistic superstitions of the people, the asceticism and soultheory of the Jains, and the pantheistic speculations of the poets of the pre-Buddhistic Upanishads, still retained the belief in transmigration.