An 'Upanisad' is a teaching session with a guru, and these thirteen texts, the 'Principal Upanisads', form a series of philosophical discourses between teacher and student that question the inner meaning of the world. Composed from around the eighth century BCE, the Upanisads have been central to the development of Hinduism, and explore the central doctrines of rebirth, karma, overcoming death, and achieving detachment, equilibrium and spiritual bliss. Speaking to the reader in direct, unadorned prose or lucid verse, they embody humanity's perennial search for truth and knowledge.