Approving or reformulating the theories of the thinkers of anterior period, is not at all a fault of the thinkers or philosophers of the posterior period, because thought development is a continuous and dynamic process. Thought cannot mature within a short span of time. Thought need time to perfect itself. Thought of a single person or a handful of people may develop rapidly at an astonishing pace. But thought of a society or a circle of thinkers cannot progress that much easily. As new ideas arise in the circle/society, supporters and detractor will sprang up here and there. Then there happen vigorous debates and challenges among them. Amidst these heated contests and controversies, the theory or theories that stand high will pass to the next generations. There such theories will undergo the same course of contest and controversies which its predecessor idea/ideas had underwent. This process will continue endlessly unless the culture or civilization never interrupted by an internal or external disasters. And fortunately Indian civilization, culture and philosophy are unaltered since its inception and have not succumbed to any external threats so far. It successfully overcame every obstacles posed by the detractors.
All of the Indian philosophical streams are not exclusive to each another. But in fact, they are in a mesh. They are interrelated in the most natural way because they have co-existed and co-operated since the dawn of Indian civilization. They had undergone severe debates with one another to test the calibre and agility of their doctrine. Though this debates, they came to realize the weak and strong points of their own system and in order to fix the drawbacks of the system, they have not refused to accept certain contents of the opposite theories, into their own system. Then a synthesis follows. This is a common trend existed in the Indian philosophic circle. Common ideas are to be found in every philosophic doctrine. The differences between them, being minor, did not damage the underlying unity of the Indian philosophical systems.
It is often believed that Sri Buddha totally despised the Vedic teaching and set up a new religion. But to say, Buddha set up a new religion, is totally baseless. Of course Buddha vehemently opposed the sacrificial outlook of certain Vedic rituals, but at the same time he was very much inclined to the knowledge based portions and teachings of that Vedic collection known as ‘The Upanishads’. Buddha has not uttered even a word against the Upanishads. In fact, most of the Buddha’s teaching and doctrines can be traced back to the Upanishads. Both the Upanishads and Buddha were similar in their opposition to certain ritualistic practices and the main doctrine of Upanishads being ‘tatvamasi’, do not draw strict boundary between different varnas.
As we see in the following pages, many of the teachings of Sri Buddha are derived from the Upanishads. It is quite non-controversial because nobody can formulate a full-fledged doctrine, like that of Sri Buddha, which has no anterior existence, either in part or full, in the other religious or secular texts. In fact, Sri Buddha explained the teachings of Upanishads in a new outlook and terminology. The core teaching of Buddha is almost akin to the Upanishads teachings.
Dependent origination (Pratitya-samutpada):-
Sri Buddha always tried to avoid giving affirmative or negative answers to certain (metaphysical) questions in order to avoid the extremes of eternalism and annihilationism, and to walk strictly through the Middle Way. As an example, for the question ‘Does the self exist after death or does it not exist?’, Buddha gave a thick silence as the reply because he knew that if he give ‘Yes’, it will interpret as promoting the ‘Eternalism’. On the other hand, if he give ‘No’, he will be promoting the annihilation theory. So he remained silent.
Buddha knew that what all exist in this mundane world, does not exist ultimately, and also, they did not non-exist totally. Then what is happening to them? The answer of Buddha was ‘they change from one state to another state continuously’. In other words, they are always ‘becoming’. Depending on the previous conditions, new one arises. And after that another one arises. In short, everything in the mundane world is in dependent nature and thus devoid of ultimate existence. They are expressible only in Name and Form (nama-rupa). This was the position of Buddha regarding the mundane world. He always tread through the Middle Way, which effectively denied eternal and annihilation theories.
Among Upanishads, the famous ‘Madhu vidya’ is in the oldest Brihadaranyaka Upanishad. It is taught by the sage Dydhyach. Though this teaching is elaborate in the Upanishad, this doctrine’s origin is in Rigveda. ‘Madhu-vidya’ teaching is multifarious. But the main theme is that, everything in the world is interconnected with one another. Let us quote from the commentary of Brihadaranyaka Upanishad by Sankaracharya.
“Because there is mutual helpfulness among the parts of the universe including the earth, and because it is common experience that those things which are mutually helpful spring from the same cause, are of the same genus and dissolve into the same thing, therefore this universe consisting of the earth etc., on account of mutual helpfulness among .its parts, must be like that. This is the meaning which is expressed in this section…”
Another learned scholar comments upon Madhu-vidya as follows:-
“…… Sage Dadhyach who is introduced in the Brihadaranyaka, as having held the doctrine of the mutual interdependence of things, because all of them are indissolubly connected in and through the Self. To quote from the ‘History of Indian Philosophy Volume II’, all things are in mutuum commercium, because they are bound together by the same vinculum substantiate, namely, the Self. The earth, says Dadhyach, is the honey of all beings, and all beings are the honey of the earth, just because the same lustrous and immortal Self inhabits them both. The fire is the essence of all things, and all things are the essence of fire, just because the immortal self is the essence of both. Similarly, are the wind, the sun, the space, the moon, the lightning, the thunder, the ether, and even law, truth, and humanity the essence of all things whatsoever, and all things are the essence thereof, inasmuch as the same law, the same element, the same indissoluble bond connects them both…”
It is a repeating theme in Upanishads that phenomenal world (and the objects exist there) has its seat in Brahman. Brahman (with Maya) is its cause and all others are its effect. And this cause and effect is different (in the case of Nama-Rupa) and same (in the case of Essence). By knowing the nature of Gold (cause), we can grasp the nature of every ornament (effect) that is made up of Gold. In the same way, by knowing the Brahman (cause), we can understand everything arise from it (effect). All that arose from Brahman exist only as just nama-rupa, name and form. Nothing in the external world has independent existence. Instead they depend on each other and live in a relative existence. Everything has its ultimate existence in the transcendental/nirguna Brahman only.
This is one of the natural outcomes of dependent origination theory. As per dependent origination theory, there is no ‘is’ or ‘is not’, no ‘being’ or ‘not being’, but only ‘becoming’. Everything in this world is changing continuously. Since a permanent and unchanging Atman cannot fulfill the ‘constant flux’ parameter, dependent origination theory give birth to the no-atman (Anatman) theory. According to this, there is no permanent entity, in the phenomenal world, that does not undergo change. This was Buddha’s firm doctrine. But there is a doubt shrouded here. Does Buddha propound the ‘anatman theory’ regarding the relative, mundane world only? Or was he applying this theory for both the mundane and trans-mundane (transcendental) plane?
If Buddha’s position was former, i.e, there is no unchanging principle named Jivatman (or simply Atman) in human (relative plane of reality), then we must say that it is in accord with the Upanishads, if we admit that the central theme of the Upanishads is non-duality between Jivatman (Atman) and Paramatman ( Brahman). In fact, the Upanishads also propound that there is no ultimate individual soul (Jivatman/Atman) in human. Our thought about an unchanging entity like Atman in us and presumption that it is the true ultimate reality/Atman, is due to the avidya or ignorance in us. Buddha also suggested that there is no Atman inside us permanently. What we feel as Atman is a collective idea that our bodily functions give birth (i.e, Soul is a term that we gives for the combined operation of five skandas) and this thought can be annulled by practicing eight fold path and realizing the four noble truths. Here Buddha clearly admits that ‘people may feel something, like an Atman, in them and they may experience this thing as unchanging’. That is, Buddha was not rejecting that ‘the people will not feel anything like Atman in them’. Instead Buddha was asserting that ‘the people may feel something like a Jivatman in them, but that conclusion is utterly wrong’. This was Buddha’s position and this is similar with the Upanishad teaching.
The Upanishads says that the thought about an individual Soul (Jivatman/Atman) in human, is a product of avidya/ignorance. By acquiring knowledge and practicing meditation, the people can get rid of the ignorance and then subsequently from the clutch of individual soul concept. He then realizes the supreme soul, Brahman. Likewise Buddha advocated his followers that ‘there is no real Atman inside body and if they feel so, they have to practice the noble eight fold path to get rid of that feeling’. The similarity in the stands of the Upanishads and Sri Budhha is indeed clear.
Now come to the former stand. Was Buddha advocating that there is no supreme soul (Paramatman or any such equivalent concept) beyond the realm of mundane world, by rejecting the individual Atman? Actually, the rejection of the individual soul does not warrant the rejection of the supreme soul, especially since the Buddha have asserted many times that he had attained a highest level of existence, which is difficult to comprehend, beyond the realm of logic and only wise can attain. Furthermore the state of Nirvana is oft said to be akin to the Brahman, though opposition views are also raised. Yet both the Brahman and Nirvana is a state where the aspirant can enjoy supreme bliss. The path that leads to this bliss is also somewhat same in both traditions. Upanishad lays importance to austerity, knowledge, discrimination, reflection (reasoning) and meditation. Buddhist way also includes many of these in a different style like understanding the four noble truths, practicing the noble eight fold path, meditation, self control, etc.
There are practical difficulties to reject an ultimate reality because relative, by default, indicates the existence of an Absolute. Without an absolute, relative cannot exist and sustain. While Buddha admit the changing character of the external world, he must have posited an absolute, without which relative cannot sustain. On the other hand if we think, all that exists is relative only, then we cannot distinguish and recognize relative as relative. Further more if there is no ultimate reality, then a Bhikshu will always be in the loopof Samsara; he will never attain Nirvana.
Yamakami Sogen points to the ultimate reality that Buddhists posits in later times,
“While condemning as rank heresy, the theories of a universal creator and of an individual soul (hinatman), Buddhism not only acknowledges the permanence of the noumenal ego, but actually enjoins its adherents to train themselves in such a manner as to be able to attain union with the Great Soul of the universe, the technical term for which is Mahatman. The locus classicus for this injunction is a well known passage in Asanga’s Mahayanasutralankara sastra where it is recommended to the aspirant to Buddhahood to look upon the Universe as a mere conglomeration of conformations (samskaras), devoid of an ego and fraught with suffering, and to take refuge from the bane of individualism in the mightily advantageous doctrine of Mahatman.”
Also Buddha is often said to be propounded that, Buddha has attained a state of existence which is difficult to comprehend for the common man, who are un-wise. But Buddha is not ready to explain the details because of the metaphysical character of the ‘existence’. Buddha’s teachings usually confines to the experimental world.
 The Buddha’s opposition to the Vedas is regarding the animal sacrifices contained in the texts. Buddha has approved that ‘the original and unaltered text of the Vedas as Apaurusheya’.
“The Buddhists have been equally deceived with Brahmins, in the estimate they have formed of the character of the Rishis. The power attributed to these revered sages by Buddhists is scarcely inferior to that of the Arhats, which we shall have to notice at greater length by and bye. By Sinhalese authors, they are represented as being possessed of superhuman attributes. In seeking to obtain Nirvana, it is great advantage of having been a Rishi in a former birth…… But Buddha denied that the Brahmans were then in the possession of the real Veda. He said that it was given in the time of Kasyapa (a former supreme Buddha) to certain Rishis, who, by the practise of severe austerities, had acquired the power of seeing Divine Bliss. They were Attako, Vamako, Vamadevo, Wessamitto, Yamataggi, Angiraso, Bharaddwajo, Wasetto, Kassapo and Bhagu. The Vedas that were revealed to these Rishis were subsequently altered by Brahmans, so that they are now made to defend the sacrifice of animals, and to oppose the doctrine of Buddha. It is on account of this departure from the truth, that Buddha refused to pay them any respect.”
—“The Sacred books of Buddhists compared with history and modern science” by Robert Spence Hardy. Page 30-31.
 Many non-Brahmin philosophers were prominently featured in the Upanishads and in Buddhist Sangha. The Upanishad teaching that ‘in the ultimate reality level, everyone is same’ had influenced the elites of that period and we hear sudra king Janasruti getting philosophical knowledge from the ascetic Rakva in Chandogya Upanishad and Sathyakama Jabala, even belongs to an unclear lineage, gets knowledge from the guru.
 Brihadaranyaka Upanishad. Section V. This is a long chapter and cannot quote here.
 Rigveda I.116.12
 Brihadaranyaka Upanishad, Sankarabhashya. Section 5.
 A constructive survey of Upanishadic Philosophy – R D Ranade. Page 52.
 This becomes a major concern especially as the Upanishads propose two states of reality; Relative reality (like external world), and a non-dual Ultimate reality, Brahman. As per the Upanishads, everything in relative reality has no independent existence of their own and their existence is rooted in the absolute reality, Brahman. So… was Buddha saying that there is no relative reality, called Jivatman, or ultimate reality, called Paramatman or Brahman? Or was he saying just, there is no relative reality, Jivatman?
 There are scholars, who argue that the Upanishads propose not the non-dual nature of Jivatman and Paramatman, but duality between them. But this article takes for granted that the Upanishadic doctrine is based on non-duality.
 Material form, sensations, perceptions, volition and consciousness are five Skandas in Buddhism.
 “The Tathagata sometimes taught that the atman exists and at other times he taught that the atman does not exist. When he preached that the atman exists and is to be the receiver of misery or happiness in the .successive life as the reward of its own Karma, his object was to save men from falling into the heresy of Nihilism (Uccheda-vada). When he taught that there is no atman in the sense of a creator or perceiver or an absolutely free agent, apart from the conventional name given to the aggregate of the five skandas, his object was to save men from falling into the opposite, heresy of Eternalism (Sasvata-vada). Now which of these two views represents the truth? It is doubtless the doctrine of the denial of atman. This doctrine, which is so difficult to understand, was not intended by Buddha for the ears of those whose intellect is dull and in whom the root of goodness has not thriven. And why? Because such men by hearing the doctrine of Anatman would have been sure to fall into the heresy of Nihilism. The two doctrines were preached by Buddha for two very different objects. He taught the existence of atman when he wanted to impart to his hearers the conventional doctrine; he taught the doctrine of anatman when he wanted to impart to them the transcendental doctrine.” – Prajnaparamita Sastra, Nagarjuna.
“The existence of the atman and of the Dharmas (i.e, of the Ego and of the phenomenal world) is affirmed in the Sacred Canon only provisionally and hypothetically, and never in the sense of their possessing a real and permanent nature.” -- Dharmapala in his commentary on the Vijnanamatra-sastra. (Both quotes citing from ‘Systems of Buddhistic thought’, by Yamakami Sogen.)
 Brahman reflecting on avidya is the Jiva, while Brahman reflecting on Maya is the Isvara.
 Brahma Jwala Sutta.
 Systems of Buddhistic Thought – Yamakami Sogen. Page 24.
 “… Into the mind of the exalted one, while he tarried, retired in solitude, came this thought: ‘I have penetrated this deep truth, which is difficult to perceive and difficult to understand, peace giving, sublime, which transcends all thought, deeply significant, which only the wise can grasp… For man, who moves in an earthly sphere, and has his place and finds his enjoyment in an earthly sphere, it will be very difficult to grasp this matter…” – Mahavagga. i.,5,2.