The Upanishads are a collection of philosophical Hindu scriptures which are commentaries on the Vedic texts. There are perhaps around 200 canonical Upanishads, the number varying from one compilation to another, though there are 10 principal Upanishads, which are considered Shruti—revealed by the divine as opposed to written strictly by human agency. The oldest Upanishads—Brhadaranyaka and Chandogya—were composed before the appearance of Buddhism in the fifth century BCE, while the rest, many of which show Buddhist influence, were written thereafter. Each of the ten principal Upanishads is associated with one of the four Vedas. The later Upanishads are largely reworkings of the principal Upanishads. The Upanishads are highly mystical and abstract, allowing for a wide range of interpretation.
The foremost contribution of the Upanishads to Hindu thought and human philosophy is the idea that the individual self (atman) and the universal spirit of all reality (Brahman) are one and the same. This concept, in its various interpretations, has come to define the entirety of Hindu philosophy. Because of this, the Upanishads, along with the Bhagavad Gita, have become the most influential scriptures in Hinduism as well as the Hindu texts most widely read by non-Hindus. The speculative and abstract nature of the Upanishads has allowed a diverse array of schools to develop within the bounds of the Hindu tradition.