The Upanishadic age was characterized by a pluralism of worldviews. While some Upanishads have been deemed 'monistic', others, including the Katha Upanishad, are dualistic. The Maitri is one of the Upanishads that inclines more toward dualism, thus grounding classical Samkhya and Yoga schools of Hinduism, in contrast to the non-dualistic Upanishads at the foundation of its Vedanta school. They contain a plurality of ideas.[note 7]
Sarvepalli Radhakrishnan states that the Upanishads have dominated Indian philosophy, religion and life ever since their appearance. The Upanishads are respected not because they are considered revealed (Shruti), but because they present spiritual ideas that are inspiring. The Upanishads are treatises on Brahman-knowledge, that is knowledge of Ultimate Hidden Reality, and their presentation of philosophy presumes, "it is by a strictly personal effort that one can reach the truth". In the Upanishads, states Radhakrishnan, knowledge is a means to freedom, and philosophy is the pursuit of wisdom by a way of life.
The Upanishads include sections on philosophical theories that have been at the foundation of Indian traditions. For example, the Chandogya Upanishad includes one of the earliest known declaration of Ahimsa (non-violence) as an ethical precept. Discussion of other ethical premises such as Damah (temperance, self-restraint), Satya (truthfulness), Dāna (charity), Ārjava (non-hypocrisy), Daya (compassion) and others are found in the oldest Upanishads and many later Upanishads. Similarly, the Karma doctrine is presented in the Brihadaranyaka Upanishad, which is the oldest Upanishad.
Development of thought
While the hymns of the Vedas emphasize rituals and the Brahmanas serve as a liturgical manual for those Vedic rituals, the spirit of the Upanishads is inherently opposed to ritual. The older Upanishads launch attacks of increasing intensity on the ritual. Anyone who worships a divinity other than the Self is called a domestic animal of the gods in the Brihadaranyaka Upanishad. The Chāndogya Upanishad parodies those who indulge in the acts of sacrifice by comparing them with a procession of dogs chanting Om! Let's eat. Om! Let's drink.
The Kaushitaki Upanishad asserts that "external rituals such as Agnihotram offered in the morning and in the evening, must be replaced with inner Agnihotram, the ritual of introspection", and that "not rituals, but knowledge should be one's pursuit". The Mundaka Upanishad declares how man has been called upon, promised benefits for, scared unto and misled into performing sacrifices, oblations and pious works. Mundaka thereafter asserts this is foolish and frail, by those who encourage it and those who follow it, because it makes no difference to man's current life and after-life, it is like blind men leading the blind, it is a mark of conceit and vain knowledge, ignorant inertia like that of children, a futile useless practice. The Maitri Upanishad states,
The performance of all the sacrifices, described in the Maitrayana-Brahmana, is to lead up in the end to a knowledge of Brahman, to prepare a man for meditation. Therefore, let such man, after he has laid those fires, meditate on the Self, to become complete and perfect.—Maitri Upanishad
The opposition to the ritual is not explicit in the oldest Upanishads. On occasions, the Upanishads extend the task of the Aranyakas by making the ritual allegorical and giving it a philosophical meaning. For example, the Brihadaranyaka interprets the practice of horse-sacrifice or ashvamedha allegorically. It states that the over-lordship of the earth may be acquired by sacrificing a horse. It then goes on to say that spiritual autonomy can only be achieved by renouncing the universe which is conceived in the image of a horse.
In similar fashion, Vedic gods such as the Agni, Aditya, Indra, Rudra, Visnu, Brahma and others become equated in the Upanishads to the supreme, immortal and incorporeal Brahman-Atman of the Upanishads, god becomes synonymous with Self, and is declared to be everywhere, inmost being of each human being and within every living creature. The one reality or ekam sat of the Vedas becomes the ekam eva advitiyam or "the one and only and sans a second" in the Upanishads. Brahman-Atman and Self-realization develops, in the Upanishad, as the means to moksha (liberation, freedom in this life or after-life).
Brahman and Atman
Two words that are of paramount importance in grasping the Upanishads are Brahman and Atman. The Brahman is the ultimate reality and the Atman is individual self. Differing opinions exist amongst scholars regarding the etymology of these words. Brahman probably comes from the root brh, which means "The Biggest ~ The Greatest ~ The ALL." Brahman is "the infinite Spirit Source and fabric and core and destiny of all existence, both manifested and unmanifested and the formless infinite substratum and from whom the universe has grown". Brahman is the ultimate, both transcendent and immanent, the absolute infinite existence, the sum total of all that ever is, was, or shall be. The word Atman means the immortal perfect Spirit of any living creature, all beings, including animals and trees. Ātman is a central idea in all the Upanishads, and "Know your Ātman" their thematic focus. These texts state that the inmost core of every person is not the body, nor the mind, nor the ego, but Atman – "Soul" or "Self". Atman is the spiritual essence in all creatures, their real innermost essential being. It is eternal, it is the essence, it is ageless. Atman is that which one is at the deepest level of one's existence.
Atman is the predominantly discussed topic in the Upanishads, but they express two distinct, somewhat divergent themes. Some state that Brahman (Highest Reality, Universal Principle, Being-Consciousness-Bliss) is identical with Atman, while others state Atman is part of Brahman but not identical. This ancient debate flowered into various dual, non-dual theories in Hinduism. The Brahmasutra by Badarayana (~ 100 BCE) synthesized and unified these somewhat conflicting theories, stating that Atman and Brahman are different in some respects particularly during the state of ignorance, but at the deepest level and in the state of Self-realization, Atman and Brahman are identical, non-different.
The idea put forth by the Upanishadic seers that Atman and Brahman are One and the same is one of the greatest contributions made to the thought of the world.
The Upanishads also contain the first and most definitive explications of the divine syllable Aum, the cosmic vibration that underlies all existence. The mantra Aum Shānti Shānti Shānti, translated as "the soundless sound, peace, peace, peace", is often found in the Upanishads. The path of bhakti or "Devotion to God" is foreshadowed in Upanishadic literature, and was later realized by texts such as the Bhagavad Gita.
Two different types of the non-dual Brahman-Atman are presented in the Upanishads, according to Mahadevan. The one in which the non-dual Brahman-Atman is the all inclusive ground of the universe and another in which empirical, changing universe is a form of Maya, often translated as "illusion".
The Upanishads describe the universe, and the human experience, as an interplay of Purusha (the eternal, unchanging principles, consciousness) and Prakṛti (the temporary, changing material world, nature). The former manifests itself as Ātman (Soul, Self), and the latter as Māyā. The Upanishads refer to the knowledge of Atman as "true knowledge" (Vidya), and the knowledge of Maya as "not true knowledge" (Avidya, Nescience, lack of awareness, lack of true knowledge).
Hendrick Vroom explains, "the term Maya [in the Upanishads] has been translated as 'illusion,' but then it does not concern normal illusion. Here 'illusion' does not mean that the world is not real and simply a figment of the human imagination. Maya means that the world is not as it seems; the world that one experiences is misleading as far as its true nature is concerned." According to Wendy Doniger, "to say that the universe is an illusion (māyā) is not to say that it is unreal; it is to say, instead, that it is not what it seems to be, that it is something constantly being made. Māyā not only deceives people about the things they think they know; more basically, it limits their knowledge."
In the Upanishads, Māyā is the perceived changing reality and it co-exists with Brahman which is the hidden true reality. Maya, or "illusion", is an important idea in the Upanishads, because the texts assert that in the human pursuit of blissful and liberating Self-knowledge, it is Maya which obscures, confuses and distracts an individual.