Upanishads are those parts of Vedic literature that contains philosophic teaching. It is usually called as Vedanta (end part of Vedas). There are many Upanishads and they are composed not by a single sage, but by many. Also the beginning to the completion of Upanishad composition may span wide period; say 500 - 1000 years, minimum.
Commonly eleven Upanishads are considered as the ‘Principle Upanishads’. Yet, this is not a hard rule. It is generally held so because Sri Sankaracharya wrote commentaries for these eleven Upanishads. They are Isa, Kena, Katha, Taittiriya, Aitareya, Mundaka, Mandukya, Prasna, Chandogya, Brihadaranyaka and Svetasvatara. There are other important Upanishads outside this group, among which Kausitaki and Maitrayani Upanishads are of special important.
The main content of all of the Upanishads is ‘Brahma-vidya’. Upanishads helps the aspirant to realize Brahman, the highest and ultimate reality, with the help of a teacher. Upanishads gives only secondary importance to meditation and karma.
The Upanishads are a store house of various philosophical ideas. All of the literature and philosophies that come to prevalent, after the composition of Upanishads, in the Indian tradition, have been tremendously influenced by the Upanishads and carries Upanishadic ideas in them. Be it Hindu, Buddhist, Jainist or Ajivika literature or philosophy, a serious reader will of course find the Upanishad ideas scattered here and there, in their literature.
Here is a modest attempt to show the influence of the Upanishads on the post-Upanishadic Indian Philosophy and religious systems.
Upanishads and Samkhya philosophy:-
Samkhya philosophy is the oldest philosophy of India. It is supposed to be formed at the end period of the principle Upanishad composition. It advocates dualism and realism. It is generally believed that initially Samkhya philosophy was theistic in outlook, but later turned to atheistic. Samkhya posits intelligent purusha at one end, and the unintelligent prakriti/pradhana/avyakta on the other end. The interrelation between them so happens due to the ignorance in the Jiva, and when Jiva get enlightened by acquiring knowledge, he realizes himself as the pure Purusha.
Samkhya propose s three gunas by which all things are composed. They are Sattva, Rajas and Tamas. All of the things in world, mental and physical, are composed of these three elements. The origin of these gunas is in the Chandogya Upanishad. There, it is described as three colors and three elements; fire, water and earth.
“The red color that (gross) fire has, that is the color of (subtle) fire. That which is the white color (of the gross fire), that is of (subtle) water. That which is the black color (of the gross fire), that is the color of (subtle) earth. (Thus) vanishes the firehood of fire. All transformation has speech as its basis, and it is name only. Those which are true are the three colors alone.”
The meaning of verse is simple. External objects composed of the fire, water, earth are just names; i.e., in name and form (namarupa) only. What is the basis of all these external objects (name & forms) are the three gunas/colors. There is no doubt that Samkhya tenet of ‘sattva – rajas – tamas’ is indicated here.
Samkhya philosophers map their doctrine even back to Rigveda. There, in 10th mandala, Pradhana is mentioned as ‘unborn’, as per them.
“The waters, they received that germ primeval wherein the Gods were gathered all together.
It rested set upon the unborn's navel, that One wherein abide all things existing.”
Samkhya categories are mentioned in Katha Upanishad.
“Beyond the senses…… is the mind; beyond the mind is intellect; beyond intellect is the Great Atman; beyond the Great Atman is the Avyakta; beyond Avyakta is the great Purusha; while beyond Purusha there is nothing else.”
The hymn clearly shows leaning toward Samkhya philosophy. This will be evident if one compares the verse with Samkhya karika of Isvara Krishna. Mundaka Upanishad also contains verses that are pointing to Sankhya Philosophy. Svetasvatara Upanishad contains typical Samkhya nomenclatures like Pradhana, Avyakta, etc. These all clearly maps the Samkhya doctrine with the Upanishads.
Upanishads and Yoga system:-
Though Upanishads pay primary importance to ‘knowledge’ to realize Brahman, meditation and rituals play subordinate roles in this process. They are able to prepare the aspirant up to a particular level. Though there are verses in Katha Upanishad, which are suggestive to indicate a Yoga system in its early phases, Svetasvatara Upanishad gives ample references to a well developed Yoga system.
“Keeping the three (head, neck and chest) up straight, the body erect, and with the help of the mind, withdrawing the senses into the heart, the wise one crosses over all the fearsome waters with the boat of Brahman (Omkara)…… Moderate and disciplined in all his activities, the wise person regulates his breath (in the seat of meditation) and when it has become gentle, he breaths out through the nostrils. Like controlling a chariot drawn by wild horses, he holds the mind single pointed and alert…… Concentrate the mind in a place like a windless cave, a place that is clean, even, free from pebbles, fire or sand, which is not noisy or near a water source, a place that is pleasing to the eyes and conducive to the mind.”
After explaining the progress, and symptoms of the upcoming perfection attainment, the Upanishad describes the final goal as follows.
“Just as a metal disc covered with earth shines as full of light when cleaned well, so too the embodied being, realizing the truth of the Self, becomes non-dual, fulfilled and free from sorrow…… When the Yogi, with the mind absorbed here in meditation, realizes the Truth (Brahman) verily as the Self (Atman) like a lamp (effulgent), knowing the divine Being as unborn, eternal and free of all modifications, he is released from all bondages.”
The description of the Yoga system as expounded in the Svetasvatara Upanishad is thus.
Yogic meditation in Upanishads:-
There are several verses about the meditation practice, which is an essential element of the Yoga system, in Upanishads. A few of them are commenting upon here.
“This letter (Om), indeed, is the (inferior) Brahman (Hiranyagarbha), and this letter is, indeed, the supreme Brahman. Anybody, who, (while) meditating on this letter, wants any of the two, to him comes that… This medium is the best; this medium is the supreme (and the inferior) Brahman. Meditating on the medium, one becomes adorable in the world of Brahman.”
“There are indeed three worlds, the world of men, the world of Manes, and the world of Gods. This world of men is to be won through the son alone, and by no other rite; the world of the Manes through rites; and the world of the gods through meditation. The world of the Gods is the best of the worlds. Therefore they praise meditation.”
“It is not comprehended through the eye, nor through speech, nor through the other senses; nor is It attained through austerity or Karma. Since one becomes purified in mind through the favorableness of the intellect, therefore can one see that indivisible Self through meditation.”
We also find a definition, akin to the definition of Yoga, in Katha Upanishad.
“When the five senses of knowledge come to rest together with the mind, and the intellect, too, does not function. That state they call the highest… They consider that keeping of the senses steady as Yoga. One becomes vigilant at that time, for Yoga is subject to growth and decay.”
Since the importance of Yoga or meditation is not up to the level of knowledge, to realize Brahman, Upanishad sage is saying that the Yoga is subjected to growth and decay; one has to perform it continuously for it to grow or sustain; else, it will decay. But knowledge about Brahman is not so; once an aspirant attain Brahma-vidya, it will never fade away.
Long before the Yoga system was written down, as a treatise by Rishi Patanjali, Upanishadic sages have well versed and practiced meditation and austerities to realize the supreme reality. Today, Yoga is as prominent as Advaita Vedanta, for realizing the ultimate truth, Brahman.
Upanishads and Nyaya-Vaiseshika Pluralism:-
These realistic schools propose that there are multiple souls which are co-eternal with the God. They list many reasons for the existence of God and being realists Nyaya claims, object and (its) qualities are different, not same as like the idealists think. Nyaya have developed sixteen categories and stresses much on the syllogism to arrive in the correct knowledge. Nyaya philosophy is systematically expounded in the Nyaya-sutra of Rishi Gotama. Vatsayana, Udyotakara, Vacaspati Misra and Udayana are the major writers and commentators on this sutra and other related treatises. Nyaya and Vaiseshika systems are considered as the sister philosophical sects in Indian philosophy. Differences between these systems are meager and so, they do not invite different treatments.
As one may expect, seeds of Nyaya-Vaiseshika system is in the Upanishadic teaching. Monism and dualism, of course cannot satisfy the thinking minds, of that period. What they sees when they look around, in the phenomenal world, is a full-fledged pluralism. While monism states that there is a single substratum lies behind these all pluralist phenomena and the pluralism is an imposed aspect on this single substratum, many cannot agree with it because to know the existence of this single ultimate substratum, special intuitive knowledge is needed. From the experimental or phenomenal point of view such a single substratum cannot prove realistically, but only theoretically. (To realize this single substratum one has to transcend the realm of phenomenal existence, which is not an easy task for commoners). Hence some disagree with the supreme Brahman concept and came out with their own theories. The emergence of pluralist schools of Vaiseshika and Nyaya is here. Of these Vaiseshika is more ancient school than Nyaya.
Matters are composed of eternal atoms and we cannot destroy them, as per Rishi Kanada, the founder of Vaiseshika System. Thought Upanishadic teaching is fundamentally monistic and, transcendental idealistic, the pluralistic and realistic speculations are also commented upon in them; (or we may say that certain Upanishad passages can be interpreted as supportive to pluralism). However, in Upanishads, these comments are supposed to be from the phenomenal point of view; not from the ultimate truth’s point of view. From the ultimate truth’s standpoint, Brahman is one without a second; there is no plurality. But from the phenomenal point of view pluralistic ideas can exist and it is not in contradiction with the absolute idealism of Upanishadic teaching.
Thus Mundaka Upanishad says:
“That thing that is such is true: As from a fire fully ablaze, fly off sparks in their thousands that are akin to the fire, similarly O good-looking one, from the Imperishable originates different kinds of creatures and into It again they merge.”
Here the creation and dissolution is from and into the same thing, the Imperishable (Brahman). As long as this creation, sustenance and dissolution, have a relative and dependent (on Brahman) outlook, as stated in the Upanishad text, we may take it in the ‘real’ sense, which clearly matches to the Nyaya – Vaiseshika school’s philosophy. Roots of pluralistic ideas are also found in the Vedas.
Purva Mimamsa doctrine and Upanishads:-
Though Upanishads falls in Jnana tradition, which gives more importance to the knowledge over the ritual performance, there are passages in various Upanishads extolling the importance of karma or rituals, in Brahman realization. Yet it is very much evident that, the Upanishads considers ‘Jnana’ as superior to anything else to realize the ultimate reality, Brahman.
Mundaka Upanishad points to the importance of rituals.
“That thing that is such is true. The Karmas that the wise discovered in the mantras are accomplished variously, where the three Vedic duties get united. You perform them forever with the desire for the true results. This is your path leading to the fruits of karma acquired by yourselves…… When the fire begins set ablaze, the flame shoots up, one should offer the oblations into that part that is in between the right and the left.”
Mundaka Upanishad further says that, if agnihotra sacrifice is not performed properly, then the future seven worlds of the non-performer will be destroyed. Agnihotra rite is said to be of that much importance.
“It (agnihotra) destroys the seven worlds of that man whose agnihotra sacrifice is without Darsa and Paunamasa rites, devoid of caturmasya, bereft of Agrayana, unblest with guests, goes unperformed, is unaccomplished by Vaisvadeva rite, and is performed perfunctorily.”
Upanishad further says that performer of all prescribed sacrifices will get into the place of the lord.
“These oblations turn into the rays of the sun and taking him up they lead him, who performs the rites in these shining flames at the proper time, to where the single lord of the gods presides over all…… Saying ‘come, come’, uttering pleasing sounds such as, ‘this is your well-earned, virtuous path which leads to heaven’, and offering him adoration, the scintillating oblations carry the sacrifice along the rays of the sun.”
Ritual performance, the trademark of the Purva Mimamsa tradition, was prevalent in India from the dawn of Indian civilization. Anyone reading the Rigveda, may understand it amply. By the period of Upanishads, the importance of ritualistic tradition dented considerably, though did not vanish fully.
Uttara Mimamsa or Vedanta in Upasnishads:-
Advaita Vedanta elements in Upanishads:-
In Advaita Vedanta individual self and supreme self are one and same. Individual self is the reflection of the supreme self on avidya, which is in the Jiva. When this avidya becomes exhausted, individual self become aware of its supreme status and realize the Brahman. In short, as per Advaita Vedanta, we all are already liberated beings. But due to avidya we are not aware of it. When we acquire Brahma-vidya, we will realize our default supreme nature, or the divinity within us. This is the nutshell of Advaita Vedanta.
We may find several passages in the Upanishad collection about the non-dual nature of Atman and Brahman. In fact, though several philosophical ideas are present in the Upanishads, the prominent teaching of the Upanishads is the non-dual, absolute monism of Advaita Vedanta.
In Chandogya upanishad, Uddalaka Aruni teaches his son Svetaketu about the nature of Self as’Tat tvam asi’ or ‘You are that’.
“…… O good looking one, of this person when he departs, (the organ of) speech is withdrawn into the mind, mind into the vital force, vital force into the fire, and fire into the supreme deity…… That which is this subtle essence, all this has got That as the Self, That is Truth, That is the Self, Thou art That, O Svetaketu.”
Monism is in full force in the portion of Aruni’s teaching. This same teaching (‘Tat tvam asi’) in a varied form, can be found in the entry gate of the Oracle of Delphi in ancient Greece. There the word inscribed as ‘know thyself’!!
Brihadaranyaka Upanishad says that individual self and the supreme self (Brahman) are same, though they may appear to be different in the relative point of view; i.e, in phenomenal world.
“…… The lord of the maya (notions superimposed by ignorance) is perceived as manifold, for to Him are yoked ten organs, nay, hundreds of them. He is the organs; He is ten and thousands – many and infinite. That Brahman is without prior or posterior, without interior or exterior. This self, the perceiver of everything, is Brahman. This is the teaching.”
In Katha Upanisahad, Yama convinces Naciketas that whoever does not realize this non-dual nature of the supreme self, will take birth again and again in this world.
“What indeed is here, is there; what is there, is here likewise. He, who sees as though there is difference here, goes from death to death.”
Yama further instructs Naciketas that how the single ultimate reality, Brahman, appear as multifarious in the phenomenal world, due to the avidya of Jiva. In that way, It is untouched by the limited nature of phenomenal world.
“Just as fire, though one, having entered the world assumes separate forms in respect of different shapes, similarly, the Self inside all beings, though one, assumes a form in respect of each shape; and (yet) it is outside…… As air, though one, having entered into this world, assumes separate forms in respect of different shapes, similarly, the Self inside all beings, though one, assumes a form in respect of each shape; and (yet) It is outside.”
And after realizing the non-dual nature of Brahman, by discerning Its multifarious appearances, Atman becomes one with Brahman.
“Eternal peace is for those – and not for others – who are discriminating and who realize in their hearts Him who – being one, the controller, and the inner self of all – makes a single form multifarious.”
Upanishads are full of monistic ideas which are the basis of Advaita Vedanta philosophy.
Visishtadvaita or qualified monism:-
Upanishadic accounts of ultimate reality are often impersonal, attribute-less. But personified accounts of ultimate reality are also available, occasionally, where qualified monism may aptly fits.
In qualified monism, ultimate reality is not devoid of qualities, but with qualities. Devotion is the major element of this spiritual system. In Mundaka Upanishad, the ultimate reality Purusha is mentioned with qualities, which is the usual method of qualified Monism.
“When the seer sees the Purusha – the golden-hued creator, lord, and the source of the inferior Brahman – then the illuminated one completely shakes off both merit and demerit, becomes taintless, and attains absolute equality.”
In numerous other passages also, ultimate reality is ascribed with attributes.
Bheda vada or dualism:-
Dualism states that individual soul and supreme soul are completely different. They are neither same nor part of one in another. Yet the individual soul depends on the supreme soul for the liberation.
There are verses in the Upanishads which may interpret as pointing to the un-relatedness of individual and supreme soul. A major verse among them is in Mundaka Upanishad; two birds sitting on a tree, one of which is eating the fruits, but the other bird just looking at the first one, dispassionately. This portion has its origin in Rigveda.
“Two birds that are ever associated and have similar names, cling to the same tree. Of course, one eats the fruit of divergent tastes and the other looks on without eating…… On the same tree, the individual soul is drowned (ie stuck), as it were; and so it moans, being worried by its impotence. When it sees thus the other, the adored Lord and His glory, then it becomes liberated from sorrow…… When the seer sees the Purusha – the golden hued, creator, lord, and the source of the inferior Brahman – then the illuminated one completely shakes off both merit and demerit, becomes taintless, and attains absolute equality.”
Here, ‘eating the fruit’ symbolizes that the bird is enjoying his karmaphala for the past actions and is entangled in the riddle of birth-death. Then he sees the other bird, the Supreme self, and began to adore it, and thus coming out of the Samsara. Here individual and supreme soul is different, but individual soul depends on the supreme soul for its liberation.
Saivism and Bhakti Marga in Upanishads:-
Bhakti as a way for liberation (Moksha-marga) has existed in India from the time of Rigveda. In the hymns attributed to Varuna, we will feel the ardent devotion of the worshipper. Varuna is described as, the Sun as his eyes, sky as garment and storm as his breath. Also Varuna is harsh to the evildoers and kind if they seek penance.
“Before this Varuna may we be sinless him who shows mercy even to the sinner-
While we are keeping Aditi's ordinances. Preserve us evermore, ye Gods, with blessings.”
There are numerous other hymns that show the immense worshipping mood of the devotee towards Varuna. S Radhakrishnan has aptly said about the devotee’s mental mood towards Varuna.
“The theism of the Vaishnavas and the Bhagatavas, with its emphasis on bhakti, is to be traced to the Vedic worship of Varuna, with its consciousness of sin and trust in divine forgiveness…”
Among Upanishads, it is Svetasvatara that contains clear cut Bhakti (devotion) elements. Saivism is the predominant feature of Svetasvatara Upanishad.
“Know that nature is surely maya and the Lord of maya is Mahesvara, the supreme lord. This whole world is verily filled by His limbs…… By realizing the one Lord who presides over every womb, in whom everything exists and merges, who bestows boons, and who is self-effulgent and adorable, one attains supreme peace…… May Rudra who is the origin of all deities and the source of all their powers, who is all knowing and the Lord of the entire universe, who initially brought into being Hiranyagarbha, endow us with auspicious (noble) thoughts.”
Siva is the protector and sustainer, and by the unconditional devotion towards him, the worshipper will achieve the final goal of liberation from all bondages.
“Since you are birthless, a person who is frightened (of samsara) takes refuge in You. O Rudra, protect me always with Your face that is turned southwards.”
“Realizing the Siva to be hidden in all beings like the subtle essence of ghee that rises to the surface, knowing that God to be the one entity that encompasses the entire universe, one becomes free of all bondages.”
In Brihadaranyaka Upanishad, another level of devotion is visible; it is towards the inner ruler or Antaryamin. There Yajnavalkya praises the Antaryamin as the one who inhabit in the earth (without the earth knowing him), water (without water knowing him), in the fire, etc.
Devotion theme is present in other texts also, especially in Katha Upanishad. It is suffice to say here that the Bhakti as a way to moksha was prevalent in India from Rigvedic times and it reached in zenith in Srimad Bhagavat Gita.
Carvaka elements in Upanishads?
This is a disputed and interesting question. Can we find materialistic elements in Upanishads? Almost all opinions tend to be negative because Upanishad teaching is strictly in spiritual level, not in materialistic level. Though certain names, including Carvaka, mentioned in Upanishads, all of them are name-presentation only; it is not a teaching.
Even being so, there are certain portions which may be interpretable as carrying materialistic idea. Thus in Brihadaranyaka Upanishad the great sage Yanjavalkya is talking to his wife, Maitreyi that ‘it is for its own sake that Self perform everything’.
“He said. It is not for the sake of the husband, my dear, that he is loved, but for one’s own sake, that he is loved. It is not for the sake of wife, my dear, that she is loved, but for one’s own sake that she is loved. It is not for the sake of the sons, my dear, that they are loved, but for one’s own sake that they are loved. It is not for the sake of wealth, my dear, that it is loved, but for one’s own sake it is loved. It is not for the sake of the Brahmana, my dear that he is loved, but for one’s own sake that he is loved. It is not for the sake of the Kshatriya, my dear, that he is loved, but for one’s own sake that he is loved. It is not for the sake of worlds, my dear, that they are loved, but for one’s own sake that they are loved. It is not for the sake of Gods, my dear, that they are loved, but for one’s own sake that they are loved. It is not for the sake of beings, m dear, that they are loved, but for one’s own sake that they are loved. It is not for the sake of all, my dear, that all is loved, but for one’s own sake that it is loved. The self, my dear Maitreyi, should be realized – should be heard of, reflected on and meditated upon. By the realization of the self, my dear, through hearing, reflection and meditation, all this is known.”
This portion of Brihadaranyaka Upanishad is often pointed as supportive to the Carvaka system. ‘Whatever we do in real life is said to be for the sake of our own soul’. Though this portion does not openly propose that pleasure is the primary goal, there is ample gap to interpret the above verse as not discouraging the hedonism of Carvakas. There are some ‘pleasure seeking’ elements in the above lines, which may be in agreement with the Carvaka trait. So interpreting this verse in line with the materialism may not be totally baseless.
But, the credibility of this deductive reasoning loses its clout when the reader proceeds further with the Upanishad. Yajnavalkya’s other teachings have disputed the deducted materialistic claims very well. So interpreting this portion of Brihadaranyaka as supportive to Carvakas is neither sure nor final.
Furthermore, the ending verse of this portion is in agreement with Vedantic conception. It advices the student to ‘hear’ the teaching about the soul from a teacher, then internally perceive the truth contained in the teaching, and finally, take the assistance of meditation, to establish the knowledge about Brahman and realize It.
Even in the celebrated story of ‘Prajapati – Indra – Virocana’, at first Prajapati says to Indra and Virocana, who enquired to Prajapati about Self/Atman, that “body is the Self… When the body is well adorned, attired and clean, then Self also will be well adorned, attired and clean.”
On hearing this reply Virocana was satisfied and walked away. But Indra did not. He scrutinized the reply and again went back to Prajapati to ask further questions. In the following portion of the Upanishad, Indra comes to understand that, what actually the Self is. Here is a hint about the Carvakas. Virocana and his followers, who believed Prajapati’s answer, ‘body is the Self’, might have acted and lived accordingly.
As we saw above, materialistic elements are very weak in Upanishads. The central teaching of Upanishads is Brahma-vidya only.
 And at the end of 1500 AD, Vijnanabhikshu has revived the theistic Samkhya.
 Rigveda 10.82.6
 Katha Upanishad. I.iii.9-10
 Mundaka Upanishad. I.i.8-9
 Svetasvatara Upanishad I.8 & I.10.
 Katha Upanishad II.iv.8
 Svetasvatara Upanishad. II.8-10
 Svetasvatara Upanishad. II.11-13
 Svetasvatara Upanishad. II.14-15
 Katha Upanishad. I.ii.15-16
 Brihadaranyaka Upanishad. I.V.16
 Mundaka Upanishad. III.i.8
 Katha Upansiahd. II.iii.10-11
 A detailed account of these reason are furnished in Kishor Kumar Chakrabarti’s ‘Classical Indian philosophy of Mind: the Nyaya dualist tradition’.
 Though we say, Rishi Kanad is the founder of Vaiseshika system, that means that he gave a systematic structure for the, then prevalent, pluralistic ideas. Pluralism must have been existed even before Rishi Kanada.
 Mundaka Upanishad II.i.1
 Peculiar enough, this Upanishad also contains passages that denounce the ritual performance.
 Mundaka Upanishad. I.ii.1-2
 Mundaka Upanishad. I.ii.3
 Mundaka Upanishad. I.ii.5-6
 Chandogya Upanishad VI.8.6-7
 Brihadaranyaka Upanishad II.v.18
 Katha Upanishad. II.i.10
 Katha Upanishad. II.ii.9-10
 Katha Upanishad. II.ii.12
 Mundaka Upanishad. III.i.3
 Mundaka Upanishad. III.i.1-3
 Rigveda VIII.41
 Rigveda VII.87.7
 Indian Philosophy, Vol 1, Page 52
 Svetasvatara Upanishad. IV.10-12
 Svetasvatara Upanishad. IV.21
 Svetasvatara Upanishad. IV.16
 Brihadaranyaka Upanishad. II.IV.5
 Internal perception is often equated to ‘reflection’.
 Meditation is helpful to desist from bondage, before Brahman-realization, and keep the mind calm and quiet, after Brahman-realization. The verse is not indicating that meditation has a prominent place to realize Brahman, but may be said to enjoy a subordinate role.
 Chandogya Upanishad. VIII.8.3