[5] The Kathaka Upanishad is an important ancient Sanskrit corpus of the Vedanta sub-schools, and an influential Śruti to the diverse schools of Hinduism.

It asserts that "Atman (Self) exists", teaches the precept "seek Self-knowledge, which is Highest Bliss", and expounds on this premise like the other primary Upanishads of Hinduism.

[10] Other philosophers such as Arthur Schopenhauer praised it, Edwin Arnold rendered it in verse as "The Secret of Death", and Ralph Waldo Emerson credited Katha Upanishad for the central story at the end of his essay Immortality, as well as his poem "Brahma".

For example, a closely pronounced word Katha (Sanskrit: कथा) literally means "story, legend, conversation, speech, tale".

Nachiketa, the boy and a central character in the Katha Upanishad legend, similarly, has closely related words with roots and meanings relevant to the text.

Basham date the Katha Upanishad's composition roughly to the 5th century BCE, chronologically placing it after the first Buddhist Pali canons.

[14] Nachiketa is mentioned in the verses of chapter 3.11 of Taittiriya Brahmana, both as a similar story,[14] and as the name of one of five fire arrangements for rituals, along with Savitra, Caturhotra, Vaisvasrja and Aruna Agni.

[2][20] The style and structure suggests that some of the verses in Katha Upanishad, such as 1.1.8, 1.1.16-1.1.18, 1.1.28, among others, are non-philosophical, do not fit with the rest of the text, and are likely to be later insertion and interpolations.

[26] Nachiketa's first wish is that Yama discharge him from the abode of death, back to his family, and that his father be calm, well-disposed, not resentful and the same as he was before when he returns.

[26] For his second wish, Nachiketa prefaces his request with the statement that heaven is a place where there is no fear, no anxiety, no old age, no hunger, no thirst, no sorrow.

[27] Yama adds that along with "three Nachiketa fires", anyone who respects three bonds (with mother, father and teacher), does three kinds of karma (rituals, studies and charity), and understands the knowledge therein, becomes free of sorrow.

This is achievable through the realization of Atman-Brahman, asserts Katha Upanishad, and this essence is reminded in the Vedas through the word Om (ॐ, Aum), as stated in verses 1.2.15-1.2.16.

In final verses of the second Valli, the Katha Upanishad asserts that the knowledge of the Atman cannot be attained through intellectual effort, reason, or scriptural study alone.

[46] The third Valli of Katha Upanishad presents the parable of the chariot, to highlight how Atman, body, mind, senses and empirical reality relate to a human being.

The Katha Upanishad asserts that one who does not use his powers of reasoning, whose senses are unruly and mind unbridled, his life drifts in chaos and confusion, his existence entangled in samsara.

Those who use their intelligence, have their senses calm and under reason, they live a life of bliss and liberation, which is the highest place of Vishnu.

A similar simile is found in ancient Greek literature, such as the Parmenides, Xenophon's prologue of Prodikos, and in the Platonic dialogue Phaedrus.

The Self is hidden in all beings, asserts the Katha Upanishad; it does not show itself, but its awareness is felt by seers with agrya sukshma (subtle, more self-evident conscious, keen thinkers).

[50][52] In verse 1.3.13, Katha Upanishad states that Prajna (conscious man) should heed to the ethical precept of self-examination and self-restraint, restraining his speech and mind by the application of his Buddhi (power to reason).

It begins by stating that human body is like a Pura (Sanskrit: पुर, town, city) with eleven gates[66] that connect it to the universe.

[68] In verses 2.5.6 and 2.5.7, the Katha Upanishad discusses what happens to the Self after death, stating a variant of the premise of Karma theory that underlies major Indian religions,[69] योनिमन्ये प्रपद्यन्ते शरीरत्वाय देहिनः । स्थाणुमन्येऽनुसंयन्ति यथाकर्म यथाश्रुतम् ॥ ७ ॥[70]

Some of these Selfs enter into the womb, in order to embody again into organic beings, others assemble unto what is Sthānu (immovable things), according to their karma, according to their shrutam (श्रुतम्, knowledge, learning).

[68][69] Parts of the ideas in these first two similes of Katha Upanishad are of far more ancient origins, and found for example in Book 6, Chapter 47 of Rig veda.

[72] The sixth Valli continues the discussion of Karma and rebirth theory, sections of which Max Muller states is possibly interpolated and inserted in a later period.

यदा पञ्चावतिष्ठन्ते ज्ञानानि मनसा सह । बुद्धिश्च न विचेष्टते तामाहुः परमां गतिम् ॥ १० ॥ तां योगमिति मन्यन्ते स्थिरामिन्द्रियधारणाम् । अप्रमत्तस्तदा भवति योगो हि प्रभवाप्ययौ ॥ ११ ॥[78] Only when Manas (mind) with thoughts and the five senses stand still, and when Buddhi (intellect, power to reason) does not waver, that they call the highest path.

Charles Johnston has called Katha Upanishad as one of the highest spiritual texts, with layers of metaphors embedded therein.

To Johnston, the three nights and three boons in the first Valli of Katha Upanishad, for example, are among the text's many layers, with the three connoting the past, the present and the future.

[83][84] The American poet Ralph Waldo Emerson held Katha Upanishad highly, and wrote several poems and essays paralleling the themes in it.

For example, Elizabeth Schiltz[87] has compared "the parable of the chariot" in Katha Upanishad and Platonic dialogue "Phaedrus", noting the "remarkable similarities give rise to a great many tantalizing historical and literary questions", and adding the comment, "each provides an image of the self as the chariot, they each offer a complex moral psychology, and point toward an effective justification of the best life".

[87] Radhakrishnan notes that Katha Upanishad's discussion of "good versus pleasant" is evidence of ethical theories and philosophical longings of ancient human beings in India by 1st millennium BCE, much like those in Greek city states in Europe.