The Katha Upanishad, also titled "Death as Teacher", is one of the primary Upanishads commented upon by Adi Shankara. It contains passages that suggest contact with Buddhist ideas, so was likely composed after the fifth century BCE. It figures as number 3 in the Muktika canon of 108 Upanishads. It consists of two chapters (adhyayas), each divided into three sections (vallis) that contain between 15 and 29 verses (slokas) apiece.
Shlokas on YouTube:
The Maandookya Upanishad belongs to the Atharva Veda. It is one of the shortest Upanishads, containing only 12 verses. It's length however, has no bearing on the knowledge contained within it. In fact, the Muktikopanishad says that if a person cannot afford to study all of the Upanishads, it is enough to read just the Maandookya Upanishad.
The Maandookya Upanishad refers to four states of consciousness – wakefulness, dreaming, dreamless sleep and the Absolute or Self, explained through the principle of AUM. AUM consists of three elements – 'a', 'u' and 'm'. The 'a' corresponds to the state of wakefulness (jagrata), wherein one is aware of sensory experiences. The 'u' corresponds to the state of dreaming sleep (svapna), where the mind is the only thing that is awake. The 'm' corresponds to the state of dreamless sleep (susupti), where only the Atman is aware of this experience. The silence that follows the chant of AUM corresponds to the Fourth state (turiya).
Turiya, which literally means "The Fourth" is the ultimate state of consciousness. It is the three preceding levels, yet it transcends them all. It is consciousness itself.
Shlokas on YouTube:
The Mundak Upanishad is said to have been imparted by Lord Brahma to his eldest son Atharva. Thus it is a part of the Atharvaveda. The name of the Upanishad comes from the word mund (to shave), metaphor being one who has been liberated form his ignorance.
The Upanishad contains three chapters, each of which is called a 'mundak'. These chapters are further divided into sub sections, called 'khanda'. The Upanishad is in the form of a mantra, containing a total of 64 verses.
The Mundak Upanishad preaches that there are two types of knowledge (vidyaa). 'Paraa vidyaa', the higher one, is the knowledge of the Brahman. 'Aparaa vidyaa', the lower one, is the knowledge of the physical world. The Upanishad goes on to say that all knowledge collected by mankind – phonetics, grammar, etymology, even the teachings in the Vedas, is aparaa vidyaa. Only the true knowledge of the Supreme Being is paraa vidyaa.
As the spider sends forth and draws in its thread, as plants come up from the earth, as hair grows on a living person, the universe arises from the Imperishable.
(Mundak Upanishad I – 1:7)
Shlokas on YouTube:
The Prashna Upanishad (or Prashnopanishad) is one of the 'primary' Upanishads commented upon by Adi Shankara. It is a 'Mukhya' Upanishad, associated with the Atharvaveda. It figures as number 4 in the Muktika canon of 108 Upanishads.
The Prashna Upanishad consists of six questions and their answers. In fact, the name itself (prashna), means 'question'.
As narrated in the beginning of this Upanishad, six pupils interested in understanding Brahman come to sage Pippalada and request him to clarify their spiritual doubts. Each of them asks one question to Pippalada and the answers form a chapter in the Upanishad.
Shlokas on YouTube: