Cycle of Birth & Death
According to the Hindu sage Adi Shankaracharya, the world - as we ordinarily understand it - is like a dream: illusory. To be trapped in samsara (the cycle of birth and death) is a result of ignorance of the true nature of our existence.
It is ignorance (avidya) of one's true self that leads to ego-consciousness, grounding one in desire and a perpetual chain of reincarnation. The idea is intricately linked to action (karma), a concept first recorded in the Upanishads. Every action has a reaction and the force determines one's next incarnation. One is reborn through desire: a person desires to be born because he or she wants to enjoy a body, which can never bring deep, lasting happiness or peace (ānanda).