The Upanishads are those parts of Vedic literature that contains philosophical 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 Upanishads composition may span a 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 guru/teacher. The Upanishads gives only secondary importance to meditation, bhakti and karma.
The Upanishads are a store house of various philosophical ideas. All of the literature and philosophies that came to prevalent, after the composition of Upanishads, have been tremendously influenced by the Upanishads and carries Upanishadic ideas in them. Be it Hindu, Buddhist, Jainist or Ajivika literature or philosophies, a serious reader will of course find the Upanishad ideas scattered here and there, in these literature.
Here is a modest attempt to show the influence of the Upanishads on the post-Upanishadic Indian Philosophies and religious systems.
The Upanishads and Samkhya Philosophy:-
Samkhya philosophy is the oldest philosophy of India. It is supposed to be formed in the end period of the composition of principle Upanishads. It advocates dualism and realism. It is generally believed that initially Samkhya philosophy was theistic in outlook, but later turned to atheistic. Samkhya posits an intelligent Purusha at one end, and the unintelligent Prakriti/Pradhana/Avyakta on the other end. The interrelation between them happens due to the ignorance in the Jiva, and when Jiva get enlightened by acquiring knowledge, Jiva realizes himself as pure Purusha.
Samkhya propose s three gunas by which all things are composed. They are Satva, Rajas and Tamas. All things in the world, mental and physical, are composed of these three elements. The origin of these gunas is in the Chandogya Upanishads, if not earlier. 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 and 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 the 10th mandala, Pradhana is mentioned as ‘unborn’.
“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.”
This hymn clearly shows leaning towards 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 to the Upanishads.
Upanishads and Yoga System:-
Though, the Upanishads pay primary importance to ‘knowledge’ to realize Brahman, meditation, bhakti and rituals play subordinate roles in this process. They are helpful to rise the capacity of aspirant, to know the ultimate reality, upto a particular level. Though there are verses in Katha Upanishad, which are suggestive of indicating Yoga System in its early phase, 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 descriptions of the Yoga system as expounded in the Svetasvatara Upanishad are 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, the 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 related austerities to realize the supreme reality.
 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
Featured Image Credit: – https://www.thoughtco.com/the-upanishads-basics-1770575