The fundamental doctrine common to all the Upanishads of the Atharva-veda is developed by most of them in various special directions. They may accordingly be divided into four categories which run chronologically parallel with one another, each containing relatively old and late productions. The first group, as directly investigating the nature of the Atman, has a scope similar to that of the Upanishads of the other Vedas, and goes no further than the latter in developing its main thesis. The next group, taking the fundamental doctrine for granted, treats of absorption in the Atman through ascetic meditation (yoga) based on the component parts of the sacred syllable om. These Upanishads are almost without exception composed in verse and are quite short, consisting on the average of about twenty stanzas. In the third category the life of the religious mendicant (sannyasin), as a practical consequence of the Upanishad doctrine, is recommended and described. These Upanishads, too, are short, but are written in prose, though with an admixture of verse. The last group is sectarian in character, interpreting the popular gods Çiva (under various names, such as Içana, Maheçvara, Mahadeva) and Vishnu (as Narayana and Nrisimha or "Man-lion") as personifications of the Atman. The different Avatars of Vishnu are here regarded as human manifestations of the Atman. The oldest and most important of these Atharvan Upanishads, as representing the Vedanta doctrine most faithfully, are the Mundaka, the Praçna, and to a less degree the Mandukya. The first two come nearest to the Upanishads of the older Vedas, and are much quoted by Badarayana and Çankara, the great authorities of the later Vedanta philosophy. They are the only original and legitimate Upanishads of the Atharva. The Mundaka derives its name from being the Upanishad of the tonsured (munda), an association of ascetics who shaved their heads, as the Buddhist monks did later. It is one of the most popular of the Upanishads, not owing to the originality of its contents, which are for the most part derived from older texts, but owing to the purity with which it reproduces the old Vedanta doctrine, and the beauty of the stanzas in which it is composed. It presupposes, above all, the Chhandogya Upanishad, and in all probability the Brihadaranyaka, the Taittiriya, and the Kathaka. Having several important passages in common with the Çvetaçvatara and the Brihannarayana of the Black Yajurveda, it probably belongs to the same epoch, coming between the two in order of time. It consists of three parts, which, speaking generally, deal respectively with the preparations for the knowledge of Brahma, the doctrine of Brahma, and the way to Brahma.