Ātman (; Sanskrit: आत्मन्) is a Sanskrit word that means inner self, spirit, or soul. In Hindu philosophy, especially in the Vedanta school of Hinduism, Ātman is the first principle: the true self of an individual beyond identification with phenomena, the essence of an individual. In order to attain Moksha (liberation), a human being must acquire self-knowledge (atma Gyan). For the different schools of thought, self-realization is that one's true self (Jīvātman) and the ultimate reality (Brahman) are: completely identical (Advaita, Non-Dualist), completely different (Dvaita, Dualist), or simultaneously non-different and different (Bhedabheda, Non-Dualist + Dualist).
The six orthodox schools of Hinduism believe that there is Ātman in every living being (jiva). This is a major point of difference with the Buddhist doctrine of Anatta, which holds that there is no soul or self.
Ātman (Atma, आत्मा, आत्मन्) is a Sanskrit word which means "essence, breath, soul." It is derived from the Proto-Indo-European word *h₁eh₁tmṓ (a root meaning "breath" with Germanic cognates: Dutch adem, Old High German atum "breath," Modern German atmen "to breathe" and Atem "respiration, breath", Old English eþian). It can also be linked to the Greek word "atmos", from which the word atmosphere is derived. 
Ātman, sometimes spelled without a diacritic as atman in scholarly literature, means "real self" of the individual, "innermost essence", and soul. Atman, in Hinduism, is considered as eternal, imperishable, beyond time, "not the same as body or mind or consciousness, but... something beyond which permeates all these". In Advaita vedanta, it is "pure, undifferentiated, self-shining consciousness," the witness-consciousness which observes all phenomena yet is not touched by it.
The earliest use of the word Ātman in Indian texts is found in the Rig Veda (RV X.97.11). Yāska, the ancient Indian grammarian, commenting on this Rigvedic verse, accepts the following meanings of Ātman: the pervading principle, the organism in which other elements are united and the ultimate sentient principle.
Other hymns of Rig Veda where the word Ātman appears include I.115.1, VII.87.2, VII.101.6, VIII.3.24, IX.2.10, IX.6.8, and X.168.4.
Ātman is a central idea in all of the Upanishads, and "know your Ātman" is their thematic focus. These texts state that the core of every person's self is not the body, nor the mind, nor the ego, but Ātman, which means "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 the deepest level of one's existence.
The Brihadaranyaka Upanishad describes Atman as that in which everything exists, which is of the highest value, which permeates everything, which is the essence of all, bliss and beyond description. In hymn 4.4.5, Brihadaranyaka Upanishad describes Atman as Brahman, and associates it with everything one is, everything one can be, one's free will, one's desire, what one does, what one doesn't do, the good in oneself, the bad in oneself.
That Atman (self, soul) is indeed Brahman. It [Ātman] is also identified with the intellect, the Manas (mind), and the vital breath, with the eyes and ears, with earth, water, air, and ākāśa (sky), with fire and with what is other than fire, with desire and the absence of desire, with anger and the absence of anger, with righteousness and unrighteousness, with everything — it is identified, as is well known, with this (what is perceived) and with that (what is inferred). As it [Ātman] does and acts, so it becomes: by doing good it becomes good, and by doing evil it becomes evil. It becomes virtuous through good acts, and vicious through evil acts. Others, however, say, "The self is identified with desire alone. What it desires, so it resolves; what it resolves, so is its deed; and what deed it does, so it reaps.
This theme of Ātman, that the soul and self of every person and being is the same as Brahman, is extensively repeated in Brihadāranyaka Upanishad. The Upanishad asserts that this knowledge of "I am Brahman", and that there is no difference between "I" and "you", or "I" and "him" is a source of liberation, and not even gods can prevail over such a liberated man. For example, in hymn 1.4.10,
Brahman was this before; therefore it knew even the Ātma (soul, himself). I am Brahman, therefore it became all. And whoever among the gods had this enlightenment, also became That. It is the same with the sages, the same with men. Whoever knows the self as “I am Brahman,” becomes all this universe. Even the gods cannot prevail against him, for he becomes their Ātma. Now, if a man worships another god, thinking: “He is one and I am another,” he does not know. He is like an animal to the gods. As many animals serve a man, so does each man serve the gods. Even if one animal is taken away, it causes anguish; how much more so when many are taken away? Therefore it is not pleasing to the gods that men should know this.
Along with the Brihadāranyaka, all the earliest and middle Upanishads discuss Ātman as they build their theories to answer how man can achieve liberation, freedom and bliss. The Katha Upanishad, for example, explains Atman as the imminent and transcendent innermost essence of each human being and living creature, that this is one, even though the external forms of living creatures manifest in different forms. For example, hymn 2.2.9 states,
As the one fire, after it has entered the world, though one, takes different forms according to whatever it burns,
so does the internal Ātman of all living beings, though one, takes a form according to whatever He enters and is outside all forms.
Katha Upanishad, in Book 1, hymns 3.3 to 3.4, describes the widely cited analogy of chariot for the relation of "Soul, Self" to body, mind and senses. Stephen Kaplan translates these hymns as, "Know the Self as the rider in a chariot, and the body as simply the chariot. Know the intellect as the charioteer, and the mind as the reins. The senses, they say are the horses, and sense objects are the paths around them". The Katha Upanishad then declares that "when the Self [Ātman] understands this and is unified, integrated with body, senses and mind, is virtuous, mindful and pure, he reaches bliss, freedom and liberation".
The Chandogya Upanishad explains Ātman as that which appears to be separate between two living beings but isn't, that essence and innermost, true, radiant self of all individuals which connects and unifies all. Hymn 6.10 explains it with the example of rivers, some of which flow to the east and some to the west, but ultimately all merge into the ocean and become one. In the same way, the individual souls are pure being, states the Chandogya Upanishad; an individual soul is pure truth, and an individual soul is a manifestation of the ocean of one universal soul.
Ātman is a key topic of the Upanishads, but they express two distinct, somewhat divergent themes. Some teach that Brahman (highest reality; universal principle; being-consciousness-bliss) is identical with Ātman, while others teach that Ātman is part of Brahman but not identical to it. This ancient debate flowered into various dual and 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 (advaita). This synthesis overcame the dualistic tradition of Samkhya-Yoga schools and realism-driven traditions of Nyaya-Vaiseshika schools, enabling it to become the foundation of Vedanta as Hinduism's enduring spiritual tradition.
All major orthodox schools of Hinduism – Nyaya, Vaisesika, Samkhya, Yoga, Mimamsa, and Vedanta – accept the foundational premise of the Vedas and Upanishads that "Ātman exists". Jainism too accepts this premise, although it has its own idea of what that means. In contrast, both Buddhism and the Charvakas deny that there is anything called "Ātman/soul/self".
Knowing Ātman, also referred to as self-knowledge, is one of the defining themes of all major orthodox schools of Hinduism, but they diverge on how. In Hinduism, self-knowledge is the knowledge and understanding of Atman, what it is, and what it is not. Hinduism considers Atman as distinct from the ever-evolving individual personality characterized with Ahamkara (ego, non-spiritual psychological I-ness Me-ness), habits, prejudices, desires, impulses, delusions, fads, behaviors, pleasures, sufferings and fears. Human personality and Ahamkara shift, evolve or change with time, state the schools of Hinduism; while, Atman doesn't. Atman, state these schools, is the unchanging, eternal, innermost radiant self that is unaffected by personality, unaffected by ego of oneself, unaffected by ego of others; Atman is that which is ever-free, never-bound, one that seeks, realizes and is the realized purpose, meaning, liberation in life. Puchalski states, "the ultimate goal of Hindu religious life is to transcend individually, to realize one's own true nature", the inner essence of oneself, which is divine and pure.
Philosophical schools such as Advaita (non-dualism) see the "spirit/soul/self" within each living entity as being fully identical with Brahman. The Advaita school believes that there is one soul that connects and exists in all living beings, regardless of their shapes or forms, and there is no distinction, no superior, no inferior, no separate devotee soul (Atman), no separate god soul (Brahman). The oneness unifies all beings, there is divine in every being, and that all existence is a single reality, state the Advaita Vedanta Hindus. In contrast, devotional sub-schools of Vedanta such as Dvaita (dualism) differentiate between the individual Atma in living beings, and the supreme Atma (Paramatma) as being separate.
Advaita Vedanta philosophy considers Atman as self-existent awareness, limitless and non-dual. To Advaitins, the Atman is the Brahman, the Brahman is the Atman, each self is non-different from the infinite. Atman is the universal principle, one eternal undifferentiated self-luminous consciousness, the truth asserts Advaita Hinduism. Human beings, in a state of unawareness of this universal self, see their "I-ness" as different from the being in others, then act out of impulse, fears, cravings, malice, division, confusion, anxiety, passions, and a sense of distinctiveness. To Advaitins, Atman-knowledge is the state of full awareness, liberation, and freedom that overcomes dualities at all levels, realizing the divine within oneself, the divine in others, and in all living beings; the non-dual oneness, that God is in everything, and everything is God. This identification of individual living beings/souls, or jiva-atmas, with the 'one Atman' is the non-dualistic Advaita Vedanta position.
The monist, non-dual conception of existence in Advaita Vedanta is not accepted by the dualistic/theistic Dvaita Vedanta. Dvaita Vedanta calls the Atman of a supreme being as Paramatman, and holds it to be different from individual Atman. Dvaita scholars assert that God is the ultimate, complete, perfect, but distinct soul, one that is separate from incomplete, imperfect jivas (individual souls). The Advaita sub-school believes that self-knowledge leads to liberation in this life, while the Dvaita sub-school believes that liberation is only possible in after-life as communion with God, and only through the grace of God (if not, then one's Atman is reborn). God created individual souls, state Dvaita Vedantins, but the individual soul never was and never will become one with God; the best it can do is to experience bliss by getting infinitely close to God. The Dvaita school, therefore, in contrast to monistic position of Advaita, advocates a version of monotheism wherein Brahman is made synonymous with Vishnu (or Narayana), distinct from numerous individual Atmans. Dvaita school, states Graham Oppy, is not strict monotheism, as it does not deny existence of other gods and their respective Atman.
In the Akshar-Purushottam Darshan school of Vedant, the atman, referred to as the jiva, is defined as a distinct, individual soul, i.e. a finite sentient being. Jivas are bound by maya, which hides their true self, which is characterized by eternal existence, consciousness, and bliss. There are an infinite number of jivas. They are extremely subtle, indivisible, impierceable, ageless, and immortal. While residing within the heart, a jiva pervades the entire body by its capacity to know (gnānshakti), making it animate. It is the form of knowledge (gnānswarūp) as well as the knower (gnātā). The jiva is the performer of virtuous and immoral actions (karmas) and experiences the fruits of these actions. It has been eternally bound by maya; as a result, it roams within the cycle of birth and death. Birth is when a jiva acquires a new body, and death is when it departs from its body. Just as one abandons one's old clothes and wears new ones, the jiva renounces its old body and acquires a new one.
Ātman, in the ritualism-based Mīmāṃsā school of Hinduism, is an eternal, omnipresent, inherently active essence that is identified as I-consciousness. Unlike all other schools of Hinduism, Mimamsaka scholars considered ego and Atman as the same. Within Mimamsa school, there was divergence of beliefs. Kumārila, for example, believed that Atman is the object of I-consciousness, whereas Prabhakara believed that Atman is the subject of I-consciousness. Mimamsaka Hindus believed that what matters is virtuous actions and rituals completed with perfection, and it is this that creates merit and imprints knowledge on Atman, whether one is aware or not aware of Atman. Their foremost emphasis was formulation and understanding of laws/duties/virtuous life (dharma) and consequent perfect execution of kriyas (actions). The Upanishadic discussion of Atman, to them, was of secondary importance. While other schools disagreed and discarded the Atma theory of Mimamsa, they incorporated Mimamsa theories on ethics, self-discipline, action, and dharma as necessary in one's journey toward knowing one's Atman.
The Vaisheshika school of Hinduism, using its non-theistic theories of atomistic naturalism, posits that Ātman is one of the four eternal non-physical substances without attributes, the other three being kala (time), dik (space) and manas (mind). Time and space, stated Vaiśeṣika scholars, are eka (one), nitya (eternal) and vibhu (all pervading). Time and space are indivisible reality, but human mind prefers to divide them to comprehend past, present, future, relative place of other substances and beings, direction and its own coordinates in the universe. In contrast to these characteristics of time and space, Vaiśeṣika scholars considered Ātman to be many, eternal, independent and spiritual substances that cannot be reduced or inferred from other three non-physical and five physical dravya (substances). Mind and sensory organs are instruments, while consciousness is the domain of "atman, soul, self".
The knowledge of Ātman, to Vaiśeṣika Hindus, is another knowledge without any "bliss" or "consciousness" moksha state that Vedanta and Yoga school describe.
Early atheistic Nyaya scholars, and later theistic Nyaya scholars, both made substantial contributions to the systematic study of Ātman. They posited that even though "self/soul" is intimately related to the knower, it can still be the subject of knowledge. John Plott states that the Nyaya scholars developed a theory of negation that far exceeds Hegel's theory of negation, while their epistemological theories refined to "know the knower" at least equals Aristotle's sophistication. Nyaya methodology influenced all major schools of Hinduism.
The Nyaya scholars defined Ātman as an imperceptible substance that is the substrate of human consciousness, manifesting itself with or without qualities such as desires, feelings, perception, knowledge, understanding, errors, insights, sufferings, bliss, and others. Nyaya school not only developed its theory of Atman, it contributed to Hindu philosophy in a number of ways. To the Hindu theory of Ātman, the contributions of Nyaya scholars were twofold. One, they went beyond holding it as "self evident" and offered rational proofs, consistent with their epistemology, in their debates with Buddhists, that "Atman exists". Second, they developed theories on what "Atman is and is not". As proofs for the proposition "self/soul exists", for example, Nyaya scholars argued that personal recollections and memories of the form "I did this so many years ago" implicitly presume that there is a self that is substantial, continuing, unchanged, and existent.
Nyayasutra, a 2nd-century CE foundational text of Nyaya school of Hinduism, states that the soul is a proper object of human knowledge. It also states that soul is a real substance that can be inferred from certain signs, objectively perceivable attributes. For example, in book 1, chapter 1, verses 9 and 10, Nyayasutra states
Ātman, body, senses, objects of senses, intellect, mind, activity, error, pretyabhava (after life), fruit, suffering and bliss are the objects of right knowledge.
Desire, aversion, effort, happiness, suffering and cognition are the Linga (लिङ्ग, mark, sign) of the Ātman.
Book 2, chapter 1, verses 1 to 23, of the Nyayasutras posits that the sensory act of looking is different from perception and cognition–that perception and knowledge arise from the seekings and actions of Ātman (soul). The Naiyayikas emphasize that the Ātman has qualities, but is different from its qualities. For example, desire is one of many qualities of the Ātman, but the Ātman does not always have desire, and in the state of liberation, for instance, the Ātman is without desire.
The concept of Ātman in Samkhya, the oldest school of Hinduism, is quite similar to one in Advaita Vedanta school. Both Samkhya and Advaita consider the ego (asmita, ahamkara) rather than the Ātman to be the cause of pleasure and pain. They both consider Ātman as self, soul that is innermost essence of any individual being. Further, they both consider self-knowledge as the means of liberation, freedom and bliss. The difference between Samkhya and Advaita is that Samkhya holds there are as many Atmans as there are beings, each distinct reality unto itself, and self-knowledge a state of Ipseity. In contrast, the monism theme of Advaita holds that there is one soul, and that the self of all beings are connected and unified with Brahman. The essence and spirit of everything is related to each self, asserts Advaita Vedanta, and each Atman is related to the essence and spirit of everything; all is one; self is Brahman and Brahman is self. Samkhya asserts that each being's Atman is unique and different.
The Yogasutra of Patanjali, the foundational text of Yoga school of Hinduism, mentions Atma in multiple verses, and particularly in its last book, where Samadhi is described as the path to self-knowledge and kaivalya. Some earlier mentions of Atman in Yogasutra include verse 2.5, where evidence of ignorance includes "confusing what is not Atman as Atman".
Avidya (अविद्या, ignorance) is regarding the transient as eternal, the impure as pure, the pain-giving as joy-giving, and the non-Atman as Atman.
In verses 2.19-2.20, Yogasutra declares that pure ideas are the domain of the soul, the perceivable universe exists to enlighten the soul, but while the soul is pure, it may be deceived by complexities of perception or its intellect. These verses also set the purpose of all experience as a means to self-knowledge.
The seer (soul) is the absolute knower. Though pure, modifications are witnessed by him by coloring of intellect.
The spectacle exists only to serve the purpose of the Atman.
In Book 4, Yogasutra states spiritual liberation as the stage where the yogin achieves distinguishing self-knowledge, he no longer confuses his mind as his soul, the mind is no longer affected by afflictions or worries of any kind, ignorance vanishes, and "pure consciousness settles in its own pure nature".
The Yoga school is similar to the Samkhya school in its conceptual foundations of Ātman. It is the self that is discovered and realized in the Kaivalya state, in both schools. Like Samkhya, this is not a single universal Ātman. It is one of the many individual selves where each "pure consciousness settles in its own pure nature", as a unique distinct soul/self. However, Yoga school's methodology was widely influential on other schools of Hindu philosophy. Vedanta monism, for example, adopted Yoga as a means to reach Jivanmukti – self-realization in this life – as conceptualized in Advaita Vedanta.
The Atman theory in Upanishads had a profound impact on ancient ethical theories and dharma traditions now known as Hinduism. The earliest Dharmasutras of Hindus recite Atman theory from the Vedic texts and Upanishads, and on its foundation build precepts of dharma, laws and ethics. Atman theory, particularly the Advaita Vedanta and Yoga versions, influenced the emergence of the theory of Ahimsa (non-violence against all creatures), culture of vegetarianism, and other theories of ethical, dharmic life.
The Dharmasutras and Dharmasastras integrate the teachings of Atman theory. Apastamba Dharmasutra, the oldest known Indian text on dharma, for example, titles Chapters 1.8.22 and 1.8.23 as "Knowledge of the Atman" and then recites,
There is no higher object than the attainment of the knowledge of Atman. We shall quote the verses from the Veda which refer to the attainment of the knowledge of the Atman. All living creatures are the dwelling of him who lies enveloped in matter, who is immortal, who is spotless. A wise man shall strive after the knowledge of the Atman. It is he [Self] who is the eternal part in all creatures, whose essence is wisdom, who is immortal, unchangeable, pure; he is the universe, he is the highest goal. – 184.108.40.206-7
Freedom from anger, from excitement, from rage, from greed, from perplexity, from hypocrisy, from hurtfulness (from injury to others); Speaking the truth, moderate eating, refraining from calumny and envy, sharing with others, avoiding accepting gifts, uprightness, forgiveness, gentleness, tranquility, temperance, amity with all living creatures, yoga, honorable conduct, benevolence and contentedness – These virtues have been agreed upon for all the ashramas; he who, according to the precepts of the sacred law, practices these, becomes united with the Universal Self. – 220.127.116.11
The ethical prohibition against harming any human beings or other living creatures (Ahimsa, अहिंसा), in Hindu traditions, can be traced to the Atman theory. This precept against injuring any living being appears together with Atman theory in hymn 8.15.1 of Chandogya Upanishad (ca. 8th century BCE), then becomes central in the texts of Hindu philosophy, entering the dharma codes of ancient Dharmasutras and later era Manu-Smriti. Ahimsa theory is a natural corollary and consequence of "Atman is universal oneness, present in all living beings. Atman connects and prevades in everyone. Hurting or injuring another being is hurting the Atman, and thus one's self that exists in another body". This conceptual connection between one's Atman, the universal, and Ahimsa starts in Isha Upanishad, develops in the theories of the ancient scholar Yajnavalkya, and one which inspired Gandhi as he led non-violent movement against colonialism in early 20th century.
यस्तु सर्वाणि भूतान्यात्मन्येवानुपश्यति । सर्वभूतेषु चात्मानं ततो न विजुगुप्सते ॥६॥
यस्मिन्सर्वाणि भूतान्यात्मैवाभूद्विजानतः । तत्र को मोहः कः शोक एकत्वमनुपश्यतः ॥७॥
स पर्यगाच्छुक्रमकायमव्रणम् अस्नाविरँ शुद्धमपापविद्धम् । कविर्मनीषी परिभूः स्वयम्भूःयाथातथ्यतोऽर्थान् व्यदधाच्छाश्वतीभ्यः समाभ्यः ॥८॥
And he who sees everything in his atman, and his atman in everything, does not seek to hide himself from that.
In whom all beings have become one with his own atman, what perplexity, what sorrow, is there when he sees this oneness?
He [the self] prevades all, resplendent, bodiless, woundless, without muscles, pure, untouched by evil; far-seeing, transcendent, self-being, disposing ends through perpetual ages.
Buddhists do not believe that at the core of all human beings and living creatures, there is any "eternal, essential and absolute something called a soul, self or atman". Buddhists reject the concept and all doctrines associated with atman, call atman as illusion (maya), asserting instead the theory of "no-self" and "no-soul". Buddhism, from its earliest days, has denied the existence of the "self, soul" in its core philosophical and ontological texts. In its soteriological themes, Buddhism has defined nirvana as that blissful state when a person realizes that he or she has "no self, no soul".
Hindus believe in Atman. They hold that at the core of all human beings and living creatures, there is "eternal, innermost essential and absolute something called a soul, self that is Atman." The Panchakosha system defined in the Taittiriya Samhita clearly distinguishes between Atman and Anatman. In this system, the Atman is said to be enveloped in the 5 layers called Panchakosha which are Anātman (Hinduism) or the non-self. However, within the diverse schools of Hinduism, there are differences of opinion on whether souls are distinct, whether a supreme soul or god exists, whether the nature of Atman is dual or non-dual, how to reach moksha– the knowledge of self that liberates one to blissful content state of existence, and whether moksha is achievable in this life (Advaita Vedanta, Yoga) or is achievable only in after-life (Dvaita Vedanta, Nyaya). However, despite the diversity of ideas and paths in different schools of Hinduism, unlike Buddhism, the foundational premise of Hinduism is that "soul/self exists", and there is bliss in seeking self, knowing self, and self-realization.
While the Upanishads recognized many things as being not-Self, they felt that a real, true Self could be found. They held that when it was found, and known to be identical to Brahman, the basis of everything, this would bring liberation. In the Buddhist Suttas, though, literally everything is seen is non-Self, even Nirvana. When this is known, then liberation – Nirvana – is attained by total non-attachment. Thus both the Upanishads and the Buddhist Suttas see many things as not-Self, but the Suttas apply it, indeed non-Self, to everything.
Buddhist texts chronologically placed in the 1st millennium of the Common Era, such as the Mahayana tradition's Tathāgatagarbha sūtras suggest self-like concepts, variously called Tathagatagarbha or Buddha nature. These have been controversial ideas in Buddhism, and "eternal self" concepts have been generally rejected. In modern era studies, scholars such as Wayman and Wayman state that these "self-like" concepts are neither self nor sentient being, nor soul, nor personality. Some scholars posit that the Tathagatagarbha Sutras were written to promote Buddhism to non-Buddhists.
In Theravada tradition, the Dhammakaya Movement in Thailand teaches that it is erroneous to subsume nirvana under the rubric of anatta (non-self); instead, nirvana is taught to be the "true self" or dhammakaya. Similar interpretations have been put forth by the then Thai Sangharaja in 1939. According to Williams, the Sangharaja's interpretation echoes the tathāgatagarbha sutras. The Dhammakaya Movement teaching that nirvana is atta (atman) in 1999, has been criticized as heretical in Buddhism by Prayudh Payutto, a well-known scholar monk, who added that 'Buddha taught nibbana as being non-self". This dispute on the nature of teachings about 'self' and 'non-self' in Buddhism has led to arrest warrants, attacks and threats.
According to Johannes Bronkhorst, a professor of Indology specializing in early Buddhism and Hinduism, while there may be ambivalence on the existence or non-existence of self in early Buddhist literature, it is clear from these texts that seeking self-knowledge is not the Buddhist path for liberation, and turning away from self-knowledge is.
The Atman concept and its discussions in Hindu philosophy parallel with psuchê (soul) and its discussion in ancient Greek philosophy. Eliade notes that there is a capital difference, with schools of Hinduism asserting that liberation of Atman implies "self-knowledge" and "bliss". Similarly, the self-knowledge conceptual theme of Hinduism (Atman jnana) parallels the "know thyself" conceptual theme of Greek philosophy. Max Müller summarized it thus,
There is not what could be called a philosophical system in these Upanishads. They are, in the true sense of the word, guesses at truth, frequently contradicting each other, yet all tending in one direction. The key-note of the old Upanishads is "know thyself," but with a much deeper meaning than that of the γνῶθι σεαυτόν of the Delphic Oracle. The "know thyself" of the Upanishads means, know thy true self, that which underlies thine Ego, and find it and know it in the highest, the eternal Self, the One without a second, which underlies the whole world.