In the body of the Isha Upanishad a large portion is devoted to avidya and vidya, what is called ignorance and knowledge. And you might say that this is the profoundest Upanishad on the subject of ignorance and knowledge, and their interrelationship, and the synthesis of avidya and vidya. In fact the whole of the Upanishad is a tremendous synthesis; at every step there is a contrast of two opposites and the synthesis: tena tyaktena bhunjitha is one of the first contrast that you get in the Isha Upanishad (I, n°1, line2): "you renounce and you enjoy". So the synthesis between renunciation and enjoyment is the very starting point and the very first verse of the Isha Upanishad and throughout.
avidyayā mṛtyuṃ tīrtvā'mṛtamaśnute ||
avidya is the method by which you cross over the death but it is by vidya that you enjoy the immortality. So, this kind of a contrast between avidya and vidya and the need of one with the other ...if you pursue only avidya, you enter into darkness; but if you pursue only vidya, you enter into a greater darkness. That is why you should pursue both avidya and vidya, and then when you do that, you attain to immortality. This is another synthesis that you find.
If you take the Kena Upanishad, you find a synthesis of approach to the ultimate Reality by exploring the knowledge by senses, knowledge through the life processes, and knowledge through the mind: pranasya pranah. "There is a Life behind life, there is a Mind behind the mind and there is sense behind the sense”; strotasya srotam, chakshuh chakshuh, "There is an Ear behind the ear and there is the Eye behind the eye". So this reconciliation of the lower means of knowledge and the higher means of knowledge, this is the whole theme of the Kena Upanishad.
And as you go on with Upanishad after Upanishad, you find varieties of knowledge, as in Kathopanishad the knowledge that is given: what happens to one when one dies. Is death really the death? The entire knowledge, the secret of the soul, the secret of soul's passage through the body which dies, and the passage of the soul towards immortality, that whole secret knowledge is given by Yama to Nachiketas in the Katha Upanishad. Again the emphasis upon the knowledge!
If you come to Chhandogya Upanishad, you have first the conversation between Sanat kumar and Narada, where Narada, possessed of all kinds of knowledge which he describes first of all to his teacher, and if you see the list of the disciplines of knowledge that he describes, there is nothing that he does not know and yet he says, "Yet, I am not free from sorrow, therefore give me that knowledge by which I can be liberated from sorrow." And then Sanat kumar gives that knowledge by which sorrow can be transcended. Or you hear the story of Satyakama jabali and when he attains the knowledge, the knowledge of the Brahman blows on his face, as the teacher himself points out to him. Or you hear the conversation between Aruni and Shvetaketu. And Shvetaketu is a young man possessed of all knowledge that his teacher had already possessed and therefore possessed of the belief that there is nothing left for him to know, or even that he surpasses now his own fatheras far as knowledge is concerned. This is the description of Shvetaketu when he returns from his teacher's house. And then Aruni asks the question, "What is it knowing by which everything can be known?" And it is this question which develops into a dialogue and by giving various kinds of analogies, the knowledge that is imparted is: tat tvam asi, ( chand.U. Vl. 8. 7.) "Thou art that".
Or you come to Brihadaranyaka Upanishad and the dialogues between Yajnavalkya and Maitreyi. Or you come to Mundaka Upanishad or Mandukya Upanishad. The Mandukya Upanishad(n°3,4;5) gives the description of the state of jagrata, waking consciousness; the description of the dream state, the svapna avastha; the state of sleep, shushupati avastha ; and a fourth state, the turiya avastha. And a secret knowledge of how aum corresponds to these three states, and how at the end of ma there is still something left which is turiya, correspondent of turiya avastha.
So in this way, the Upanishads even while synthesizing Karma, Jnana and Bhakti, the emphasis is not upon that synthesis. Again in the Veda the emphasis was upon the gods and the man, and their relationship, not that this again is absent.
In the Isha Upanishad we have the worship of sun or Pushan, worship of Agni; in Kena Upanishad, we find reference to Agni, Vayu and Indra. So references to gods is not absent, but you can see definitely a kind of a departure from the earlier preoccupation with the gods and the Upanishadic occupation with something much different from the gods. Even the gods of the Vedas and the gods of the Upanishads are not on the same plane. The gods of the Upanishads like Indra, Varuna and Agni, can forget the Divine, in the Vedas, gods never forget the Divine. In this parable of the gods in the Kena Upanishad, these gods had won the battle and they believed that they had won -, and that is why the Supreme manifests itself, to remind them. And even then, they are not reminded, it is by the intervention of Uma, the Hemvati , who reveals to them that it is the Supreme Lord who is behind them, it is He who has brought them the victory. So you can see the difference between the gods in the Vedas and the gods in the Upanishads. But even then, Kena Upanishad (II, n°1, line2) itself says: Yadasya tvam yadasya deveshu. There are two ways by which you can approach the Divine, you can approach the Divine through the gods yadasya deveshu, or you can approach the Divine - Yadasya tvam, or you can approach through yourself. You go within yourself, that is you go deeper and deeper in you, and conquer the kingdom of your body, life and mind by the powers of the soul; or else you enlarge yourself and see the movement of gods on the Earth, gods in the antariksha, gods in the sky, gods of all the levels of existence and you discover behind them the Supreme Lord. So, Yadasya tvam yadasya deveshu. And there is a synthesis of these two powers of knowledge in the Kena Upanishad.