The Vedas deal with rituals and worship, which imply a dualism between the worshipper and the object worshipped. Vedanta spells out the principle of Advaita (non-duality). It is interpreted in different ways, but the real basis of the Advaitic principle is Ekatma bhava, that is the feeling that there is only one Atma pervading everywhere and non else. Advaita darsanam Jnanam (Wisdom lies in the perception of oneness). The Upanishad preach this oneness, based on the concept of unity in diversity. Upa means near, ni represents nishtha and shath means sit. Upanishad means that one should sit near the preceptor to acquire the Supreme Spiritual Wisdom.

The Upanishad originated during different periods of time. That is why we find that the teachings of the different Upanishad are not based on the circumstances obtaining at one particular time, but they are applicable universally at all times as they teach only what is vital for the welfare of humanity. (SSS Vol.26, p. 360)