The senses of action are twin-born with those of knowledge and so their idea is similar to that of the senses of knowledge, and all that, applies to the one applies almost equally to the other in the Upanishads.
Thus we see that the Upanishads examine in considerable detail the principal ideas appertaining to the different systems of Hindu Philosophy. We have now to see what they have to say with regard to knowledge and action as the goals of life.
In this connection we have observed that Vedanta regards both knowledge and action as two aspects of the same energy of the Soul; whereas the remaining systems hold that, while action may be necessary so long as a person lives, the ultimate goal is its cessation, and that is possible only by means of knowledge. We might say that each system of though accepts the necessity of action in proportion to its nearness to Vedanta, and of knowledge in proportion to its distance from it.
The Goal of Life in Vedanta: We have observed that knowledge and action are a joint goal of life in Vedanta; and so we are told that both knowledge and action determine our birth; that the living Soul has to enjoy the fruit of actions, but when he has known the God, he is freed from all the fetters; and that we overcome death through good actions, and obtain immortality through knowledge.
The Goal of Life in Yoga: Yoga, as we have explained, is almost identical to Vedanta. In both action, conceived as a Sacrifice, is necessary; but Yoga is based on Buddhi, which is characterised by certainty of knowledge; hence it regards knowledge as its final goal and so we are told that Understanding (Buddhi) performs all sacrifice and sacred acts; but, as the special characteristic of Buddhi is knowledge, it is said that "those who have sought the Self by penance, abstinence and knowledge (all characteristic of Buddhi) ... gain the home of the spirits, the immortal, ... from whence they do not return, for it is the end".
The Goal of Life in the Vaisesika: The Vaisesika examines all life in the light of the Mind and its character, Desire. Action is regarded as less, and knowledge more, necessary than in Yoga, and the final goal of life is to escape from the trammels of Mind and Desire and so we are told that the "Mind alone is the cause of bondage and liberty for men; if attached to the world, it becomes bound; if free from the world, that is liberty. ... The Mind must be restrained in the Heart till it comes to an end; that is knowledge that is liberty". and then it is said, that "a man is like this or like that, according as he acts and according as he behaves, so will he be. ... A person consists of desires. ... To whatever object a man's own Mind is attached, to that he goes strenuously together with his deed. But the man who does not desire, who, not desiring, free from desires, is satisfied in his desires, or desires the Self only, he goes to Brahman".
The Goal of Life in Nyaya: Nyaya, as we have observed, regards action as less, and knowledge as more, necessary than the Vaisesika; and so, while it agrees that it is necessary to perform actions so long as a person lives, it holds that we can escape from their bonds by performing them as a Sacrifice, and believes that knowledge is the final goal. Thus we are told that "when freed from the senses (the basis of Nyaya) the wise, on departing from this world, become immortal". Again it is said that good actions and acts of Sacrifice lead to Svarga (heaven) indeed, but rot to final freedom, which can be attained only by that knowledge of Brahman through which a man knows the eternal and true Person).
The Goal of Life in Sankhya: The Sankhya holds that all actions, even those of Sacrifice, must ultimately be renounced if the final goal of freedom from the bondage of Prakrti is to be achieved. Hence it regards knowledge alone as the true and final end; and in this respect that its point of view, except for the idea of Sacrifice, is almost identical to that of Nyaya.
Conclusion: Thus we see that the Upanishads contain all the principal ideas of the various systems of Hindu Philosophy, relating to Purusha, Prakrti, Soul, Buddhi, Mind, and the senses, as well as knowledge and action as the final goals and so they are, as is commonly believed, a great commentary on the hymns of the Vedas, which describe the five great creative energies of life in the form of gods and forces of nature, representing the growth and development of the organic cell, universalised into the idea of Brahmanda. A single vein of thought runs through all the Sacred Books of the Hindus, from the Vedas downwards, and it is this that is pictured in the Story of the Mahabharata.