The underlying concern of the Upanishads is the nature of Brahman, the universal soul; and the fundamental doctrine expounded is the identity of atman, or the innermost soul of each individual, with Brahman. Formulations of this doctrinal truth are stressed throughout the Upanishadic writings The Upanishads are the most important portion of the Vedas. The Upanishads contain the essence or the knowledge portion of the Vedas. The philosophy of the Upanishads is sublime, profound, lofty and soul-stirring.
The Upanishads speak of the identity of the individual soul and the Supreme Soul. They reveal the most subtle and deep spiritual truths. The Upanishadic period prepared the ground for the development of rational ideas, scientific observation and advanced study in the fields of mathematics, logic and the physical sciences. It also cleared the way for a secular and more humane approach in the realm of social relations and government. Brahmin orthodoxy and ideas of ritual purity were challenged and confronted.
The concept of god in Upanishadic thinking was quite different from the more common definition of god as creator and dispensor of reward and punishment. The Upanishadic concept of god was more abstract and subtle. It postulated the doctrine of a universal soul – that embraced all physical beings. All life emanated from this universal soul and death simply caused individual manifestations of the soul to merge or mingle back with the universal soul. The concept of a universal soul was illustrated through analogies from natural phenomenon. As the bees make honey by collecting the juices of distant trees, and reduce the juice into one form. And as these juices have no discrimination, so that they might say, I am the juice of this tree or that, in the same manner, all these creatures, when they have become merged in the True, know not that they are merged in the True. . . .”
The name of Krishna refers to the Supreme Personality of Godhead, whose spiritual form is eternal and full of knowledge and bliss. This is confirmed in the following verse of Mahabharata: The word krs is the attractive feature of the Lord’s existence, and na means spiritual pleasure. when the verb krs is added to affix na, it becomes “Krishna”, which indicates the Absolute Truth. ” The name Krishna may also be interpreted to mean the perfection of transcendental knowledge and bliss. Lord Krishna’s mercy is described in the word “aklita-karine”, which means that the Lord rescues the living entities from the five-fold miseries of material existence, and also that the Lord is so kind that He even liberated those demons fortunate enough to be personally killed by Him.