The fourth part of Vedic literature consists of the Upanishads. Each of these, too, is attached to one of the Vedic Samhitas. There are numerous Upanishads, and 108 are listed in the 17th century Muktika Upanishad, but of these only about 14 are early texts, dating to before the 3rd century BCE. The Upanishads are highly philosophical. These texts are categorized as Vedanta [Veda + anta=end], as they are both the last of the four main groups of Vedic texts, and also the most important.
These texts have many topics, but the main focus is Brahman, the eternal, beyond birth and death, unchanging, the source of all creation, yet uncreated. Brahman pervades the whole world and is in every living being. The atman or soul, is of the essence of Brahman, its nature being true consciousness and bliss. A person has the potential to realize this true nature, but trapped in things of the world, they hardly even think about it.
The early Upanishads are the Aitareya, Kaushitaki, Taittiriya, Brihadaranyaka, Chhandogya, Kena, Kathaka, Shvetashvatara, Mahanarayana, Isha, Mundaka, Prashna, Maitrayaniya, and Mandukya.