Yes. The Upanishads are parts of the shruti portion of the Vedic literature.
The late Suhotra Das was a leading disciple of A.C. Bhaktivedanta Swami Prabhupada, and he writes as follows.
The word veda means "knowledge." In the modern world, we use the term "science" to identify the kind of authoritative knowledge upon which human progress is based. To the ancient people of Bharatavarsha (Greater India), the word veda had an even more profound import that the word science has for us today.
That is because in those days scientific inquiry was not restricted to the world perceived by the physical senses. And the definition of human progress was not restricted to massive technological exploitation of material nature.
In Vedic times, the primary focus of science was the eternal, not the temporary; human progress meant the advancement of spiritual awareness yielding the soul's release from the entrapment of material nature, which is temporary and full of ignorance and suffering. Vedic knowledge is called apauruseya, which means it is not knowledge of human invention.
Vedic knowledge appeared at the dawn of the cosmos within the heart of Brahma, the lotus-born demigod of creation from whom all the species of life within the universe descend. Brahma imparted this knowledge in the form of sabda (spiritual sound) to his immediate sons, who are great sages of higher planetary systems like the Satyaloka, Janaloka and Tapaloka. These sages transmitted the Vedic sabda to disciples all over the universe, including wise men of earth in ancient times.
Five thousand years ago the great Vedic authority Krishna Dvaipayana Vyasa compiled the sabda into Sanskrit scripture (sastra) which collectively is known today as the Vedas. In the India of old, the study of the Vedas was the special prerogative of the brahmanas (the priestly and intellectual class).
There were four degrees of education in Vedic knowledge that corresponded to the four ashramas of brahminical culture (the brahmachari or student ashrama, the grhastha or householder ashrama, the vanaprastha or retired ashrama and the sannyasa or renounced ashrama).
The first degree of learning was the memorization of the Vedic Samhita, which consists of 20,000 mantras (verses) divided into four sections -- Rg, Sama, Yajur and Atharva -- that are chanted by priests in glorification of various aspects of the Supreme Being during sacrificial rituals.
The second degree was the mastery of the Brahmana portion of the Vedas, which teaches rituals for fulfillment of duties to family, society, demigods, sages, other living entities and the Supreme Lord.
The third degree was the mastery of the Aranyaka portion, which prepares the retired householder for complete renunciation.
The fourth degree was the mastery of the Upanisads, which present the philosophy of the Absolute Truth to persons seeking liberation from birth and death. The texts studied in the four stages of formal Vedic education are collectively called sruti-sastra, "scripture that is to be heard" by the brahmanas.
But sruti-sastra is not all there is to the Vedic literature. Chandogya Upanisad 7.1.2 declares that the Puranas and Itihasas comprise the fifth division of Vedic study. The Puranas and Itihasa teach the same knowledge as the four Vedas, but it is illustrated with extensive historical narrations. The fifth Veda is known as smrti-sastra ("scripture that must be remembered"). Smrti-sastra study was permitted to non-brahmanas.
Traditionally, six schools of thought propagated Vedic wisdom, each from a different philosophical perspective. Each of these perspectives or darshanas is associated with a famous sage who is the author of a sutra (code) expressing the essence of his darshana.
Vyasa's Vedanta-sutra, which carefully examines and judges the six systems of Vedic philosophy (as well as other philosophies), forms the third great body of Vedic literature after the sruti-sastra and smrti-sastra. This is known as the nyaya-sastra, "scripture of philosophical disputation." The sad-darshana (six philosophical views) are
vaisesika (atomic theory),
sankhya (analysis of matter and spirit),
yoga (the discipline of self-realization),
karma-mimamsa (science of fruitive work)
vedanta (science of God realization).
The sad-darshanas are termed astika philosophies (from asti, or "it is so"), because they all acknowledge the Veda as authoritative, as opposed to the nastika philosophies of the Carvakas, Buddhists and Jains (nasti, "it is not so"), who reject the Vedas.
Beginning with nyaya, each of the sad-darshanas in their own turn presents a more developed and comprehensive explanation of the aspects of Vedic knowledge.
Nyaya sets up the rules of philosophical debate and identifies the basic subjects under discussion: the physical world, the soul, God and liberation.
Vaisesika engages the method of nyaya or logic in a deeper analysis of the predicament of material existence by showing that the visible material forms to which we are all so attached ultimately break down into invisible atoms.
Sankhya develops this analytical process further to help the soul become aloof to matter.
Yoga, awakens the soul to its innate spiritual vision to see itself beyond the body.
Karma-mimamsa directs the soul to the goals of Vedic ritualism.
Vedanta focuses on the supreme spiritual goal taught in the Upanisads.
Originally, the six darshanas were departments of study in a unified understanding of the Veda, comparable to the faculties of a modern university. But with the onset of Kali-yuga (the Age of Quarrel), the scholars of the darshanas became divided and contentious. Some even misrepresented Vedic philosophy for their own selfish ends.
For instance, karma-mimamsa (which by 500 BC had become the foremost philosophy of the brahmana class) was misused by bloodthirsty priests to justify their mass slaughter of animals in Vedic sacrifices. But the unexpected rise of a novel non-Vedic religion challenged the power of karma-mimamsa.
This new religion was Buddhism. By 250 BC, the influence of karma-mimamsa and other darshanas had weakened considerably. When King Ashoka instituted the Buddha's doctrine as the state philosophy of his empire, many brahmanas abandoned Vedic scholarship to learn and teach nastika concepts of ahimsa (nonviolence) and sunyata (voidism).
Buddhism in its turn was eclipsed by the teachings of the Vedantist Shankara, who revived the Vedic culture all over India in the seventh century after Christ. But Shankara's special formulation of Vedanta was itself influenced by Buddhism and is not truly representative of the original vedanta-darshana taught by Vyasa.
After Shankara, vedanta was refined by the schools of great teachers (acharyas) like Ramanuja and Madhva. Having shed the baggage of Shankara's crypto-Buddhism, Vedanta philosophers soared to heights of dialectical sophistication that has been much appreciated by many Western intellectuals. It is through the dialectics of the major schools (sampradayas) of Vedanta that students can best observe the six systems of Vedic philosophy "in action."
In dialectical Vedanta, arguments are taken from nyaya, vaisesika, etc. to
1) demonstrate that Vedanta is the most comprehensive of all the darshanas, and
2) to clarify the points of controversy that arise between the different schools of Vedanta itself. Vedantic dialectics are represented in the bhasyas (commentaries) of the acharyas and the tikas (subcommentaries) of their disciples.
All possible controversial philosophical positions, including some bearing remarkable resemblance to the ideas of European philosophers, are therein proposed, analyzed and refuted. The study of the six systems of Vedic philosophy is itself a form of yoga: jnana-yoga, the yoga of theoretical knowledge. But from jnana one must come to vijnana, practical realization of the ultimate truth.
The sad-darshana are six branches of theoretical dialectics (sastratha) that twist and turn from thesis (purvapaksa) to antithesis (uttarapaksa) to synthesis (siddhanta) like the gnarled branches of a tree.
But the ways of philosophical disputation do not themselves add up to the Absolute Truth. The Absolute Truth, being transcendental, is only indirectly framed in the branches of jnana, like the rising full moon may be framed by the branches of a tree. A friend who wishes us to see the moon may first draw our attention to that tree. This may be compared to the indirect or theoretical stage of knowledge – jnana, while seeing the moon is - vijnana.
There is a straightforward path to vijnana. It is explained in the Mahabharata, Vana-parva 313.117: "Dry arguments are inconclusive. Philosophers are known for their differences of opinion. Study of the branches of the Vedas will not bring one to the correct understanding of dharma. The truth is hidden in the heart of a self-realized person. Therefore one should follow the path of such great acharyas."
The Sanskrit word acharya is derived from achara, "behavior." The great teachers of Vedanta, the acharyas, were much more than just theoreticians: by their exemplary God-conscious behavior they marked out the path of practical transcendental realization. This is the path from jnana to vijnana.
In India, the sampradayas (schools of Vedanta) established by the great acharyas are bastions of sadachara, spiritual life. Students who enter these schools cultivate divine qualities -- cleanliness, austerity, truthfulness and mercy -- without which divine knowledge cannot manifest. Cleanliness is destroyed by illicit sex, austerity is destroyed by intoxication, truthfulness is destroyed by gambling and mercy is destroyed by meat-eating; one who cannot restrain himself from these bad habits has no business calling himself a Vedantist or a yogi.
There is much enthusiasm today for theoretical yoga and mysticism, but until one follows the path of sadachara set down by the acharyas, one's inquiry into Indian spirituality will be like licking the glass of a sealed jar of honey: the higher taste (param drstva) will be missed. The Brahma-Madhva-Gaudiya Sampradaya first introduced genuine Vedanta theory and practice in the Western world in 1966, when acharya Sri Srimad A.C. Bhaktivedanta Swami Prabhupada opened the first branch of the International Society for Krishna Consciousness (ISKCON) in New York.
ISKCON now has centers world-wide. Those who wish to practice this philosophy and realize the goal of Vedanta -- the Form of the Supreme Eternal Being – should contact ISKCON.
ISKCON embodies the teachings of Sri Chaitanya Mahaprabhu. Achintya bhedabheda tattva
There are four principle acharyas in the Vaisnava conclusion, and their teachings have been synthesized by the sublime doctrine of achintya bhedabheda tattva
Achintya bhedabheda tattva is most simply described as inconceivable oneness and difference simultaneously.
The oneness is in quality and the difference is in quantity.
A common example given by Srila Prabhupada was that of a drop of water from the ocean, (the individual living entity), compared with the ocean itself, (the Absolute Truth).
In the Navadwip dham Mahatmya Srila Bhaktivinode Thakur relates the time when Sri Nityananda Prabhu took Jiva Goswami on a tour. Herein it is mentioned how Sri Chaitanya Mahaprabhu accepted two principles from each of the four Vaisnava Sampradayas.
Ramanuja of the Sri Sampradaya preached the philosophy of dasya-rasa and vasistha-advaita. Sri Chaitanya Mahaprabhu accepted the concept of unalloyed devotion free from karma and jnana - ananya bhakti, and service to the devotees, bhakta-jana-seva
Madhavacharya preached dvaita philosophy, and Sri Chaitanya Mahaprabhu accepted keval-advaita-nirasana, the complete defeat of Mayavada philosophy, and Krishna-murti-sevana, worship of the deity of Krishna, understanding it to be eternal.
Visnuswami of the Rudra sampradaya preached suddha-advaita. Sri Chaitanya Mahaprabhu accepted the sentiment of total dependence on Krishna, tadiya-sarvasva-bhava, and the path of spontaneous devotional service, raga-marga.
Nimbarka of the Kumara sampradaya preached dvaita-advaita philosophy and the two points that Mahaprabhu accepted from him were, the need of taking exclusive shelter of Radhika - ekanta-radhikasraya and the exalted mood of the gopis’ love for Krishna, gopi-bhava.
By combining these points Sri Chaitanya Mahaprabhu gives all Vaisnavas a common cause and foundation, and establishes that the living entity is simultaneously a part of Krishna, but also an eternally separate distinct individual.
How we can be part of one person and simultaneously a separate individual person is inconceivable to mundane logic and senses, but never-the-less this is the conclusion of Sri Chaitanya Mahaprabhu, and the jiva is thus eternally connected with Krishna being a part of Him, and the perception of separation is illusory, caused by maya.
While being a part of Krishna still we are not Krishna, but subordinate to Him, and can never be Him in totality. Our constitutional position is to enjoy by being enjoyed in a loving relationship with Krishna.
Thus it may seem that the philosophical and religious thought of the Vedas has evolved, but in reality the Truth has always been the Truth, and its different aspects have appeared, disappeared, and reappeared according to the times and circumstances.